Backstage at New York City's Metropolitan Opera, Deborah Voigt is being fitted for a gown to wear in Verdi's Un Ballo in Maschera. All around her, seam-stresses are tucking and pinning to bring the waistline in. Says the celebrated soprano, beaming with pride: "I'm certainly smaller than the last time I did this."

Smaller by about 120 lbs., in fact. That's how much weight Voigt, 44, has lost since early 2004, when London's Royal Opera House deemed her too big for a little black dress in a modern-day production of Strauss's Ariadne auf Naxos. It was Voigt's signature role, but the part went to a thinner singer instead.

Now Voigt is down from a size 30 to a 14. Her route: gastric surgery. She says it wasn't the dismissal that led her to have the operation—"I'd been thinking about it for a long time"—but it did free up some time in her schedule. For years she had tried to slim down. "I've done everything from Weight Watchers to fen-phen," she says, but the singer always gained back what she lost. "My knees were giving me trouble, and doing what I do was becoming too taxing. I decided now was the time to do it."

The Florida-based Voigt had the procedure last June in New York, in spite of facing not just the usual medical risks, but professional ones too: Losing weight has been known to affect some opera singers' voices. She was back onstage seven weeks after surgery, mainly to cheers. "Some people felt [her voice] lacked a little bit of the warmth she had before," says New York Times music critic Anthony Tommasini. "I didn't feel that. I felt it was a radiant and exciting sound."

Though she still calls herself "a food junkie," she says that overeating now makes her ill. "I have a baby stomach, and I'm relearning to eat." Divorced for seven years, she jokes, "I'm a cheap date. We only need one meal to share."

The metamorphosis has yet to fully sink in with Voigt, who will release a CD of American music in the fall. "I did a production in Berlin recently and wore this little negligee," she says. "The review said, 'Ms. Voigt has transformed herself into a strikingly beautiful woman.' I thought, 'Who was he looking at?' I haven't gotten my head there yet."