AT THE ENTRY to the showroom of the upscale, uptight Borgata Hotel and Casino--gritty Atlantic City's belated attempt to compete with the more relaxed luxury of the Las Vegas mega-resorts--there's a smiling, soft-toned picture of Frankie Valli. For the uninitiated potential ticket-buyer, it comes with a helpful caption. "The inspiration," it reads, "behind the hit Broadway musical 'Jersey Boys.' "

On a hot Friday evening in late August, the original Jersey boy is still out on the road with his trademark falsetto.Valli is a naturally shy and serious man with little tolerance for fools or charlatans and even less patience for the irritations and encumbrances of long-term celebrity. But his signature, onstage blend of total sincerity and Jersey nonchalance has been perfected over more than 50 years of talking to audiences just like tonight's.

"How many of you," Valli asks, peering out from the Borgata stage, his voice betraying no emotion, "have seen 'Jersey Boys'?"

He already knows the answer. Almost everyone.

That's why the Borgata showroom is sold out for a three-night stand. That's why Valli sold out the Chicago Theatre last spring and why he's returning in November. That's why his fee has gone way up--his live concerts now pull in $75,000 a night or more. That's why he's playing the flashiest casino in Jersey, instead of the second-tier joints that peddle oldie acts to slot-loving seniors or college auditoria. That's why you're reading this story.

In one of life's more bizarre turns, the Broadway telling of the remarkable life story of Frankie Valli has greatly rejuvenated the career of the actual Frankie Valli.

Valli, you might say, is suddenly riding high in the slipstream of his own life.

You'd think that would mess with a guy's head.

PUBLISHED ACCOUNTS VARY as to the real Valli's age--a subject that he has always refused to discuss and refused to discuss with me--but 70 and 72 are two of the more frequently cited numbers. The record shows that even when they were kids, the members of Valli's group, the Four Seasons, would often slice a year or so off their ages for the benefit of the even-younger girls buying their records.

As Valli well knows as he peers into the darkness of the Borgata, it's a good bet that some at the younger end of this flashy, dressed-to-the-nines Jersey crowd were conceived while such Valli-voiced hits as "My Eyes Adored You" or "Can't Take My Eyes Off You" spun on their parents' record players.

The older women in the audience--a couple of whom are twirling deftly removed undergarments in Valli's incongruously grandfatherly direction--surely remember both fearing and envying "Dawn," that good girl in love with a sexy boy from the wrong side of the tracks. Be they Polish, Irish or, especially, Italian by descent, the older men likely identified with the Four Seasons' lyrical striving to be big men in town, to get out of the ethnic, blue-collar enclaves of Newark or Queens, seduce a classy Sherry baby of their own, outdo their working-stiff dads and make something of their lives.

Like Frank Sinatra before them (but like few others), the Four Seasons were pop stars who knew how to act with sufficient sensitivity that girls would buy their records, and yet retain their macho credibility on the meaner streets of New Jersey, so the boyfriends would buy their stuff, too.

Watch Valli perform even today and you'll see a singularly fascinating mix of the masculine and feminine. He might never have sung like a man--the Four Seasons were the first major American pop group to use falsetto as their lead sound--but he always walked and talked like one. The Four Seasons were guys who made sweet music. But if you didn't like it, they were perfectly capable of taking you outside and making you like it. Or so it felt.

BOTH VALLI AND "Jersey Boys" trade on the same astounding history of musical innovation and long-lived popular success. A singer with incomparable vocal range, Valli has been recording since 1952, hitting the top of the charts seven times over an astonishing 16-year span, from "Sherry" (which he recorded with the Four Seasons) to "Grease" (recorded solo). Unlike almost all of his peers, Valli was ready and able to switch from doo-wop to pop to rock to whatever, mostly because he felt that a tune with a hook would always transcend the public's fickle love of a genre. Time and again he was proved right.

But whereas the genuine article can only talk and sing in one place at a time, Valli's theatrical life-story can easily be duplicated.

And the show "Jersey Boys," which begins performances Friday in Chicago and is expected to stay at the LaSalle Bank Theatre for a year or more, has other advantages.

The brilliant songwriter-musician Bob Gaudio, who long ago quit touring with Valli, can be put back behind the keyboards. The Four Seasons' original bass player and singer, Nick Massi, who died in 2000, can be brought back to life. Tommy DeVito, the Seasons' original lead guitarist and the most colorful, troubled and macho member of the band, can be returned to his complex prime. And--surely, the most complicated issue for Valli--the performer playing Frankie can be kept forever young, that famous range forever intact.

Not that it has slipped much. Valli can still hit most of the old high notes, and he doesn't try to go after the ones he can't. His silky turn of vocal phrase remains exquisite, in much the same way that Tony Bennett, a decade his senior, retains his craft. Nobody dares call Bennett an oldie act. Valli wants the same respect.

"By the end of the year," he tells his fans in a matter-of-fact tone, "there will be five companies of 'Jersey Boys.' One in New York. One in Chicago. One in Las Vegas. One in London. One touring."