By Stephen Holden

Seldom has a roar gone up at the Metropolitan Opera like the sustained ovation that greeted Barbara Cook when she stepped out onto its stage on January 20, 2006. This was the first time in its 123-year history that the Met officially presented a "popular" singer in concert, although other pop performers have rented the Met and Cook had appeared there in 2003 as a special guest in The Merry Widow, and afterward had performed a short set.

The 2006 concert, at which she was joined by special guests Josh Groban, Audra McDonald, and Elaine Stritch, was not a gala honoring a tottering legend from some twilit golden era but recognition of a singular, vital musical career that continues to defy gravity. At 78, an age when most lyric sopranos have long thrown in the towel, Cook was close to her artistic peak.

Her appearance at the Met was a tacit acknowledgement by this citadel of high culture that America's Broadway and cabaret popular song tradition, increasingly marginalized since the advent of rock, needs and deserves the kind of preservation and respect that has long been accorded to "classical" music. For the Met, now under ambitious new management that has vowed to blur the increasingly fuzzy lines between traditional opera and the contemporary musical theater in the Gershwin-Rodgers-Bernstein-Sondheim line, Cook's concert was an institutional turning point.

Defying the conventional arc of most performing careers, Cook's late-blooming ascendance could not have been predicted when she made her Broadway debut in 1951. In those days, she possessed a lovely light-operatic soprano and the honey blonde looks to go with it; she was a quintessential ingénue. That early phase of her career reached a pinnacle with her portrayal of Cunegonde in the 1956 musical Candide, singing Leonard Bernstein's dizzy, pirouetting coloratura spoof, "Glitter and Be Gay." Her performance, beautifully captured on the original cast album, still dazzles in its brightness and agility.

But as much as Cook is celebrated today as the quintessential voice of Broadway's golden age, she was never a bankable star of the magnitude of Mary Martin, Ethel Merman, or Julie Andrews. Born and brought up in Atlanta, she made her unremarkable Broadway debut in 1951 in Flahooley, a show (with music by Sammy Fain and Yip Harburg) that closed after 40 performances and yielded no standards.

Her reputation was built almost as much by her appearances in City Center revivals of Oklahoma (1953) and Carousel (1954 and '57 playing first Carrie Pipperidge, then Julie Jordan) as on the three most famous roles she originated. After Cunegonde came Marian the Librarian in The Music Man, which won her a Tony for best featured actress in a musical.

She is most famous, however, for playing Amalia Barash in Bock and Harnick's beloved 1963 cult hit, She Loves Me, adapted from the movie The Shop Around the Corner. Out of it emerged her longest-running signature song, "Ice Cream." The last role she originated on Broadway was Dolly Talbo in The Grass Harp (1971).

Reaching her mid-40s, Cook was forced to reinvent herself as a nightclub and concert performer, and chose as her guide and artistic partner the Broadway composer, arranger, and pianist Wally Harper. Thus began an extraordinary musical marriage that continued until his death in 2004. This new Barbara Cook was no ingénue but a full-figured lyric soprano exuding a maternal benevolence. A year after performing at Brothers and Sisters, a dingy, midtown Manhattan cabaret, where she acquired an ardent gay following, her new identity was secured in a January 1975 concert at Carnegie Hall, recorded by Columbia Records.

Under Harper's close supervision, Cook, whose voice had begun to darken, learned to swing lightly on songs like "Sweet Georgia Brown," and to cautiously investigate current folk-pop songs like Janis Ian's "Stars" and Dan Fogelberg's "Longer." As the years passed, she began to lower her keys, but the riper textures of her more mature voice played into her strengths as a peerless purveyor of long-lined lyric melodies deepened by the far-sighted compassion and understanding she brought to unabashedly romantic lyrics.

The maturing Cook expressed sadness without bitterness and embodied simplicity rooted in a rigorous musical intelligence. Her unadorned, unaffected, achingly sad rendition of Rodgers and Hart's "He Was Too Good to Me" stands as a supreme musical distillation of heartbroken empathy.

It wasn't until September 1985 that Cook entered the final triumphant phase of her career with her appearance at Lincoln Center in a legendary concert production of Sondheim's Follies, singing the role of Sally Durant. The psychologically knotty ballads "Losing My Mind" and "In Buddy's Eyes," in particular, provided Cook with a new platform for emotional expression. Since those concerts, Cook has established herself as an exemplary interpreter of Sondheim ballads: songs so deep and ruthlessly analytical about love, marriage, relationships, and the passing of time that they become artistic Rorshachs for anyone approaching them.

The emotional essence Cook pours into Sondheim is a kind of sweet forgiveness that recognizes the core of yearning beneath the composer's cynical, skeptical intellect. Cook's sweetness isn't an ingénue's syrupy appeal for puppy love but the generosity and compassion tendered by a woman who has lived through her own emotional hard times and retained an inner core of unsullied romantic memories and dreams filtered through a screen of tough love.

Since then, except for occasional songs by contemporary writers like Amanda McBroom, Cook has retreated from contemporary pop back to Broadway in shows that pay tribute not only to Sondheim but to Bock-Harnick, Rodgers and Hammerstein, and other great Broadway composers. She has also evolved into a wonderful casual storyteller with a warm, cheerfully self-deprecating sense of humor.

The core of her repertory is a pool of classic ballads that stand as her equivalents of the Puccini and Verdi arias sung by her operatic counterparts. "Wait Till You See Him," "He Was Too Good to Me," "This Nearly Was Mine," "We'll Be Together Again" (usually sung without a microphone), "Time Heals Everything," " Had Myself a True Love," "I Got Lost in His Arms," and a dozen Sondheim classics and a dozen more by other composers make up the heart of her repertory. To hear Barbara Cook sing them is to drink in as pure a musical distillation of time in a bottle as American music has produced.

Stephen Holden joined The New York Times in 1988 after writing popular music criticism for Rolling Stone and the Village Voice. Now the paper's cabaret critic, he is also one of the Times's three regular film critics. He has also reviewed and written about music for The Atlantic, Connoisseur, Playboy, Vogue, Vanity Fair, and the New Grove Dictionary of American Music. In 1980, Triple Platinum, his satirical novel about the record business, was published by Dell Books. He won a Grammy for best album notes in 1986.