The Maggio Musicale Fiorentino, where the maestro had served as artistic director from 1985 until 1991, said Bartoletti died in a Florence hospital after a long illness.
In a career that saw Bartoletti conduct well into his 80s - he directed Giacomo Puccini's "Manon Lescaut"at Florence's Teatro Comunale in February 2011 - he served as the first music director of Chicago's Lyric Opera, starting as guest conductor there in 1956, when he was relatively unknown.
Bartoletti was 30 when the then 2-year-old Lyric Opera needed a replacement conductor for Giuseppe Verdi's "Il Trovatore" in 1956. Baritone Tito Gobbi endorsed him, and Bartoletti made his American debut with the company. He conducted more than 600 performances of 55 operas in the Lyric, in his 51 years there, with his last in 2007. He served as co-artistic director with Pino Donati from 1965 till 1975, and as artistic director from 1975 till 1999.
Sir Andrew Davis, the Lyric's music director, said that when he took over the "musical reins" from Bartoletti in 2000, "I was acutely aware of the extraordinary legacy which he had left."
"Not only did he establish and maintain the great Italian opera tradition which earned the company the nickname 'La Scala West,' but also he oversaw the broadening of the repertoire" including a "remarkable range" of 20th-century operas and premieres, Davis said in a statement.
Interviewed by The Associated Press in Chicago in 2007, Bartoletti said he was returning to Florence to be with his family and announced he would never conduct outside Italy again.
He was a proponent of modern music in both Chicago and Florence.
"One of the things I am proudest of is the premiere in this city of Alban Berg's 'Wozzeck,' which I think is the great masterpiece of the 20th century," he said in that interview in Chicago.
Lyric founder Carol Fox commissioned Krzysztof Penderecki's "Paradise Lost" for the U.S. bicentennial in 1976, but the work was not completed until two years later. Bartoletti conducted the world premiere on Nov. 29, 1978.
The Lyric's general director, Anthony Freud, recalled on Sunday how Bartoletti nurtured the fledging company when he first joined. "By the time he retired as artistic director in 1999, Lyric was recognized around the world as one of the great opera companies," Freud said in a statement.
Bartoletti also conducted at La Scala , working with the legendary Milan opera house for 13 productions, starting in 1958. He was a repeat guest at the Teatro Colon in Buenos Aires.
Among the more notable albums with Bartoletti as maestro was a recording of Verdi's "Un Ballo in Maschera" with Renata Tebaldi and Luciano Pavarotti.
A wake was scheduled for Monday in the foyer of Florence's Teatro Comunale, to be followed by a funeral in Sesto Fiorentino, a Florence suburb where the maestro was born, on June 10, 1926. Bartoletti did his music studies at Florence's conservatory. The Tuscan capital conferred honorary citizenship on him in 2009.