They both have other commitments, their studio says, and were unable to make it to the red carpet.
If they had, a sloppy kiss between them, or even some discreet hand-holding would have brought the crash barriers down, such is their appeal to millions of teenagers - and, as is becoming apparent, their mothers.
Love at first bite: Twilight stars Kristen Stewart and Robert Pattinson have made millions from their on and off-screen romance
And they are an item off-screen, too, although as great Hollywood romances go, it doesn't yet have the exotic chemistry of, say, Taylor and Burton, or Brad and Angelina.
Nor has it yet developed into an intriguing partnership such as that of Humphrey Bogart and Lauren Bacall or Katharine Hepburn and Spencer Tracy.
He is a tall, floppy-haired public schoolboy from South-West London with the bewildered air of someone who has been taken completely by surprise at finding himself a heart-throb.
She is a cool brunette from California who manages to combine a sweet innocence with sulky rebelliousness.
Together they have generated more than a billion dollars (£650 million) at the box office with the first two screen adaptations of Stephenie Meyer's books in the Twilight series.
A third film, Eclipse, is released next week, and judging by the advance hysteria it has created, it could well - as the title suggests - overshadow all previous success.
OutTAKE
Kristen lives with her parents in a £5million house in California and drives a black Mini Cooper, number plate KSTEW1
Kristen lives with her parents in a £5million house in California and drives a black Mini Cooper, number plate KSTEW1
Pattinson has earned more than £11million in the past year alone, according to the influential U.S. magazine, Forbes, which has named him as one of the most powerful celebrities in the world. Kristen's earnings are £7.95million.
Their love affair - which neither will confirm, preferring to bat aside teasing questions about their relationship while holding hands and kissing passionately - has become fodder for a thousand celebrity magazine covers.
Every week they are reported to be splitting up, secretly marrying or expecting a baby. So what is the truth?
'When we're together, we understand each very well,' says 24-year-old Pattinson. 'We know what each other is thinking and we have been very close for a long time. Kristen is a wonderful girl.'
She says of him, 'He's a hunk, the sort of guy that makes any girl feel good. He's intelligent, he's modest, and - whatever else people say about our relationship - he's my best friend.'
Not that they have courted attention. Over the past two years, RPattz and KStew have seldom been pictured together in private, and photographed only when attending Twilight-related events.
When Stewart went to the premiere of Pattinson's film Remember Me, in March, she was careful not to walk down the red carpet at the same time as him.
But last month, the pair finally shared a public kiss on stage at the MTV movie awards. The joyous hormonal screaming that greeted this much-delayed public consummation was roughly the equivalent of having 30,000 vuvuzelas blown in your ear.
Twilight zone: The couple couldn't hide their chemistry as they attended the LA premiere of The Twilight Saga: Eclipse last week
'I'm still quite shy, although I've found that some women find that quite attractive.'
There is no doubt the romance is based on more than hype. Last January, they were seen house-hunting together at Ventnor on the Isle of Wight.
Kristen, who lives with her parents, is also looking for a house in Beverly Hills. And recently they were reported to have spent the night together in a bungalow in the grounds of a hotel which was once Charlie Chaplin's home in Los Angeles.
They met on the set of the first film back in 2007 when Stewart, already cast as Bella Swan, sat in on auditions to find the leading man to play her lover, Edward Cullen, a 17-year- old vampire who has been a member of the undead since 1918.
More than 3,000 clean-cut young hopefuls trooped through the studios over the course of five days. In the end, director Catherine Hardwicke whittled them down to a shortlist of 20. Stewart, however, told her that it was quite obvious who her co-star should be - 'that dishy, pretty much unknown English boy'.
When his casting was announced, fans of the Twilight books complained that Pattinson, a former male model who has wonderfully characterful eyebrows and a square jaw, was not handsome enough. More than 75,000 fans signed an online petition to say that he was not the right man to play Cullen.
But Stewart rightly judged his appeal - and won over the critics and fans. He also hit it off personally - and not just professionally - with Stewart, who had been dating another actor, named Michael Angarano.
'I pestered her until she gave in, ' says Pattinson. 'It wasn't my charm or looks - it was my persistence.'
Cast: (L-R) Meraz, Booboo Stewart, Reed, director David Slade, Lutz, Greene and Xavier Samuel attend the London premiere of Eclipse
And now Pattinson, who went to the £6,000-a-term The Harrodian school in South-West London and whose father imports vintage cars, is in the position Leonardo DiCaprio was 13 years ago as a teen idol.
Yet his only previous screen experience had been the role of the prefect Cedric Diggory in Harry Potter And The Goblet Of Fire, and his ambition was 'to be like Jack Nicholson'.
A bizarre aspiration? Maybe, but Pattinson had worshipped Nicholson his early teens. 'I love his mannerisms. I dressed talked like him. He was my idol.'
His modelling career had begun at but fizzled out at 16. He turned to stage and landed a part in a play at London's Royal Court, but was sacked during rehearsals for over-acting.
Now, riots follow in his wake. There protests when he cuts his long hair.
Only singer Justin Bieber is considered hotter - and poor Zac Efron, is yesterday's news.
OutTAKE
Robert Pattinson has written two songs for the Twilight soundtrack
Pattinson says that he still suffers from a fear of failure and feelings of inadequacy. Given that he is paid £5million a film, plus a share of the box office, and is the leading element in this hugely successful franchise, he ought to feel more secure. Robert Pattinson has written two songs for the Twilight soundtrack
But he explains that the phenomenal nature of the scrutiny he's under makes everything seem unreal. He doesn't own a car, and spends most of his time living in hotels - by choice. Spending money seems not to interest him, and he has an almost superstitious attitude to how long his fame will last.
'In one hotel I was staying at, they had really good room service with an amazing choice of dishes, but I just ate the same thing every day - chicken tikka.
'I crave home-cooked food sometimes, but I only know how to make toast. That's the way I am.'
With box- office success comes intense scrutiny of his private life. When he and Stewart appeared at a 30-minute question-and-answer session with fans, the couple's intimate body language was much-commented on.
The fans are obsessed with the two of them. 'It's extremely difficult when we are out together,' he said when they appeared at the Baftas earlier this year.
'We can't arrive anywhere at the same time because the fans go crazy. This was supposed to be a public appearance as a couple, but it's impossible. We are here together and it's a public event, but it's not easy.'
Both he and Stewart refuse to talk on the record about their relationship, but it's evident from his replies that they are very close personally, as well as professionally.
'Kristen and I are living a great adventure together,' he says. 'We are inseparable.'
In Eclipse, high school student Bella finds herself surrounded by danger after a string of mysterious killings. She is attracted to Jacob - a werewolf played by heart-throb Taylor Lautner - and the two of them share a kiss.
She has to choose between him and Edward, knowing that she might ignite a struggle between the vampires and the werewolves.
Chemistry: A scene from Eclipse, which is the third movie installment based on the novels by Stephenie Meyers
Where tje second film, New Moon, was more static, Eclipse is very much an action movie. Pattinson says he had to learn how to run properly - being filmed on a treadmill - to try to improve his form instead of just skipping and limping along.
Naturally, this played to his onscreen rival Lautner's beefcake strengths. Pattinson says: 'There are a couple of scenes where we have these confrontations where I push him around a little bit and I'm supposed to grab his shoulder. It wasn't even in the script.
'I thought I'd really scare him and grab his shoulder, that it would freak him out. So I grabbed his shoulder - but it was actually too big to get a grip on.'
The rivalry is reflected in the film's two best lines. At one point Jacob tells Edward: 'I'm hotter than you.' And Edward, seeing Jacob as usual dressed in cut-off denim shorts, with his muscled chest rippling, deadpans to Bella: 'Does he ever put on his shirt?'
That may not sound like the stuff of Oscars to non-fans, but the Twilight movies are completely critic-proof.
New Moon has taken £470 million worldwide and broke the opening-day record in America - seeing off the far more expensively marketed Batman: The Dark Knight. (The Vatican, incidentally, condemned the last film as a ' deviant moral vacuum'.)
The producers, Summit Films, certainly believe that the future is bright, as they have already planned the next, and final, two instalments.
Stephanie Meyer's final book Breaking Dawn, which includes the wedding, the first night of sexual bliss for Bella and Edward, and a birth, will be split into two parts and filmed back to back between October this year and next March.
Quite naturally Pattinson is anxious to do nothing to derail this lucrative juggernaut.
At first, he admits, he didn't want to do Twilight because he had not yet learned how to act, and wanted to wait until he was ready.
Now he says: 'I know I'm the luckiest guy on earth.'
And, as an instant megastar with a beautiful girlfriend and successful career, who would argue with that?
