HUNDREDS OF FANS lined up outside the house waiting to buy the few remaining tickets, most of them hopeful students in new blue jeans, pressed polo shirts, knapsacks and Dr. Martens work boots. A rock concert? No, a performance of Puccini's Madama Butterfly at the L.A. Opera last month.

"Going to the opera or even the ballet is kind of an important date," says John Mangum, a 21-year-old history major at UCLA and an arts editor of its Daily Bruin, who has taken his girlfriend, Regina Su, to seven opera performances this year. "It's something more than just going to see Twister."

Opera has become one of the hottest sectors in the entertainment business. According to Opera America, a trade association, the number of U.S. opera companies has climbed from 60 in the late Seventies to 109 today. Attendance is way up, especially among younger fans. Between 1982 and 1992, 18% more 18-to-24-year-olds spent nights at the opera, even though that age group shrank by 16% during that period.

Jumping on the trend, record companies, concert promoters and Madison Avenue executives have been turning their marketing machines loose. In Tokyo in late June promoter Matthias Hoffmann will kick off the first world tour of the Three Tenors--Luciano Pavarotti, Placido Domingo and José Carreras.

The tenors will do just ten shows in stadiums around the world, but those ten shows will be the biggest, most profitable live musical events of the year. Expected worldwide gross: up to $150 million from ticket sales, merchandise, TV rights and corporate sponsorships.

Stars Pavarotti, Domingo and Carreras should each receive more than $10 million. Conductor James Levine may get $5 million. Tibor Rudas, owner of the Three Tenors show and producer of the group's 1994 one-stop live event at Dodger Stadium in Los Angeles, will collect about $5 million from selling the rights to Hoffmann.

When the show hits Giants Stadium (capacity 60,000) in New Jersey on July 20, the average ticket price will be $250, with more than 4,000 tickets costing between $1,000 and $1,500 each.

Some 500 premium tickets that offer a reception with the three stars will be sold for $2,500 apiece. Compare these prices to what the Rolling Stones charged when they played Giants Stadium last year: $55.

Hoffmann, the new opera impresario, made his name promoting outdoor rock festivals in Germany in the Seventies, featuring bands like Deep Purple, Led Zeppelin and Genesis. He's betting big money that opera can draw as well or better than rock. He put up $90 million to make the tour happen, but hopes for a profit of $25 million. "If this is a promoter's fantasy, the fantasy has been very big," grins Hoffmann.

In a way Hoffmann is reviving an old tradition of big-money opera. In 1850 P.T. Barnum paid Jenny Lind--"the Swedish Nightingale"--$1,000 per night to tour America. In a two-year stint Barnum grossed $712,000--huge money in those days--more than $12 million in today's money.

In the Twenties tenors Enrico Caruso and John McCormack got modestly rich playing small amphitheaters and festivals. But in recent years opera has retreated to gilded opera houses where the ticket sales have had to be supplemented by fundraising to keep the once-popular art form alive. In the U.S. last year opera companies covered half their $434 million budgets with private donations and some government subsidies.

The official top fee for singing one night at, say, the 4,000-seat New York Metropolitan Opera is less than $15,000, although a handful of stars demand slightly more.

Slowly, however, communications technology has been creating a potential new mass audience. Satellite television and the advent of supertitles-- translations of foreign language lyrics displayed above the stage or on the backs of opera house seats--have created new fans.

Sniffing a trend, impresarios Hoffmann and Rudas arrived on the scene. Herbert Breslin, Pavarotti's manager and publicist, was formerly a public relations man in Detroit.

Opera purists are somewhat aghast. The performers don't sing an entire score, but popular arias from operas ranging from Turandot to Rigoletto--as well as operatic versions of "Singin' in the Rain" and other crowd pleasers.

Thomas Graham, a director with IMG Artists in London, describes the Three Tenors tour with a sniff: "It's a cult of celebrity--it's like watching the miniseries instead of reading the novel."

So? In December Tibor Rudas booked Pavarotti into Cleve land's 21,000-seat Gund Arena. Rudas sold $1.5 million worth of tickets to the event. Pava rotti typically nets $200,000 from such gigs.

Corporations now pay top opera stars up to $75,000 to show up at their events and sing for as little as half an hour.

FORBES estimates that in 1995 and 1996 Pavarotti will earn some $20 million in performance fees alone. Carreras and Domingo, who tend to play more traditional venues, are not far behind.

Mezzo soprano Cecilia Bartoli has appeared on late-night television promoting her debut at the Met as well as her new album, A Portrait. Kathleen Battle, another soprano, has recently released an album of popular love songs and spirituals titled So Many Stars on the Sony Classics label.

As they were in the past, opera performers are becoming popular celebrities. Tenor Roberto Alagna and his wife, soprano Angela Gheorghiu, have been featured in publications such as USAToday and Vogue. When they were married in May, New York City Mayor Rudolph Giuliani performed the ceremony.

This fall you'll start seeing promotions for Alagna and Gheorghiu's latest release, Duets & Arias, on buses, television and in magazines-- the album has a six-figure marketing budget. A few years ago that CD would have been sent to the stores with little marketing support.

Will success and big money spoil the stars? How could it? They always had big egos anyhow.