Francoise Sagan became a sensation at 18 for writing "Bonjour Tristesse," her mega-selling novel of a rich teenager's treachery toward her father's mistress. In August 1953, the bored and bohemian author, having flunked out of the Sorbonne, secluded herself in her room and typed out 200 pages that made her a celebrity for the next half-century.

Ms. Sagan, 69, who died of heart and lung ailments Sept. 24 at a hospital near her home in Normandy, spent the rest of her life trying to repeat her precocious literary success while living hedonistically amid fast and dangerous cars and men.

"Bonjour Tristesse" ("Hello Sorrow") made her independently wealthy at a young age. The book was translated into 20 languages, sold 2 million copies and was made into a 1958 movie directed by Otto Preminger and starring Jean Seberg, David Niven and Deborah Kerr.

Another of her early books, "Aimez-vous Brahms?" ("Do You Like Brahms?"), about a businessman whose mistress catches the eye of a younger man, was made into a film, "Goodbye Again" (1961), with Ingrid Bergman, Anthony Perkins and Yves Montand.

With her gamine face and a giant thirst for whiskey, shiny red Jaguars and thrilling sex, Ms. Sagan resembled a sensuous tomboy. She became a central figure in self-consciously glamorous film and literary society; she lunched, drank and otherwise frolicked with Tennessee Williams, Henry Miller, Roger Vadim, Jean-Luc Godard and Juliette Greco.

She was written about as "a notorious representative of the younger generation," a symbol of fashionable rebellion in postwar Europe. The writer Francois Mauriac described her as "a charming little monster."

Others saw little charm. She rated a papal denunciation, and a parish priest refused to officiate at her first wedding because of her books, which he considered immoral.

In dozens of books and plays, Ms. Sagan became increasingly repetitive, relying on her formula to attract a largely female readership to her tales of lust and doom amid the idle smart set. Reviewers became more cautious in their praise for the one-time enfant terrible, although few quibbled with the strengths of her first book.

In a New Yorker article, John Updike applauded "Bonjour Tristesse" for "its sparkling sea and secluding woods, its animal quickness, its academically efficient plot, its heroes and heroines given the perfection of Racine personae by the young author's innocent belief in glamour."

Ms. Sagan was born Francoise Quoirez on June 21, 1935, in southwestern France and moved with her prosperous parents to Paris after World War II. She later took her surname from a character in Marcel Proust's "Remembrance of Things Past" when her father, an industrialist, objected to the family name being sullied by her books.

Kiki, as she was called, was the youngest of three children and a troubled girl. She had a stammer and often spent days in silence as a child only to make dramatic outbursts. She once accidentally cut her lip at her father's factory and shrieked, "I don't want to die yet!"

She seemed to have constant accidents, and her behavior was less than admired by her teachers, who expelled her from several convents. She reveled in jazz joints and saw herself as a great writer before she sold a word.

She received a $150 advance on her first novel, "Bonjour Tristesse," but blew the money on a black sweater and rounds of whiskey for her friends. During the next several years, she wrote two more bestsellers, "Un Certain Sourire" ("A Certain Smile") and "Dans un Mois, Dans un An" ("In a Month, in a Year"), and a "scandalously erotic" ballet that featured a pas de deux performed in a bathroom.

With the sales from the books, she bought a mink coat for her mother and a Jaguar for herself. She suffered severe head trauma after a speeding accident involving her Aston Martin in 1957.

She was a devotee of late nights and midnight swims on the Riviera. Her hours were said to have been the cause of her divorce from her first husband, publisher Guy Schoeller, a man two decades her senior.

"I went to bed at 4 a.m., he got up at 7 to go horse riding," she told reporters in 1960. "Our decision is taken, and believe me, it makes me sad. But we could not continue."

She added: "Doubtless, I shall begin again."

She did. Her marriage to American artist Robert Westhoff ended in divorce in 1963 after she claimed he preferred his ceramics to married life. They had a son, Denis, who survives.

As she continued her prolific career, Ms. Sagan talked more about craft. She said she was not an erotic writer, choosing instead to "express everything in a single elegant phrase which allows the reader to conjure up his own sexual image."

She became known, and often parodied, for culinary aphorisms about life and love.

On sex: "To me, making love can never be a commodity, like margarine."

On marriage: "Marriage? It's like asparagus eaten with vinaigrette or hollandaise, a matter of taste but of no importance."

She professed marriage inevitably dull and at best a covenant bound by mutual toleration. She would have nothing to do with it and instead kept vibrant company.

One of her friends, Godard, encouraged Ms. Sagan, who had adapted many of her works to film and television, to try directing. She directed "Les Fougeres bleues" ("The Blue Ferns," 1977), about the passions of two couples in a hunting party.

She occasionally wrote about politics for French newspapers and issued condemnations of war and racism while chain-smoking Gauloises and downing whiskey. As her frailties became more pronounced, she was less a chic model of fast living and depicted more as a pitiable figure.

She was twice convicted of cocaine charges in the 1990s, but her sentences were suspended. In 2002, she was convicted of tax fraud in a matter involving money she received from a businessman who wanted the ear of her friend, French President Francois Mitterrand.

In the end, friends said in published reports, she was lonely, frail and financially strapped.

Her own definition of old age was when desire became impossible to fulfill, "when no more encounters are possible. But in spite of being wild in the head, some mornings your teeth chatter."

Francoise Sagan, shown in 1994, was considered by some to be a symbol of fashionable rebellion in postwar Europe. She had a fondness for whiskey and Jaguars.