Les Miserables has become the West End's longest-running musical.
The show has been seen by an estimated 51 million people
Many of the original cast attended a party to celebrate the show's 21st birthday at London's Queen's Theatre on Saturday night.
Les Miserables has chalked up roughly 38,000 professional performances worldwide since opening at the city's Barbican on 8 October 1985.
Michael Ball, who played Marius, told BBC News: "We thought we'd be looking for new jobs after the first night."
Based on the Victor Hugo novel which charts social upheaval in 19th Century France, Les Miserables was an instant hit despite mixed reviews.
LES MISERABLES IN NUMBERS
Performed in 38 countries and 223 cities
Translated into 21 languages
31 cast recordings
More than one million original London cast albums sold
Each performance uses 101 cast and crew, 392 costumes and 31 wigs
"It was due to run for 11 weeks. After the reviews we thought we would lucky to do that but the very next morning the phones were jammed," said Ball.
"It just shows the power of word of mouth and the power of the story to touch people."
Sally Frith, 41, from Kings Stanley in Gloucestershire, is thought to be the musical's biggest fan.
Saturday night's performance was the 740th time she has seen the production.
She says she has spent nearly £30,000 on trips to see the show after first watching it in 1988, since when tickets have risen from £5 to £22.50.
"I don't drink or smoke, I've got no vices apart from the theatre," she said.
"I know about 90% of the words now. I can usually tell if there's a mistake."