The 75-year-old Sills recently fell and fractured her knee, according to a statement. In addition, her husband, newspaper publisher Peter Greenough, is ailing.
"I know that the strength of her spirit and her unfailing sense of humor even when things get rough will help her through the present difficulties," said Met general manager Joseph Volpe in a statement. "She's earned the gratitude and respect of all of us, and I am sure I speak for everyone at the Met in wishing her the best."
During her tenure, Sills helped to raise millions of dollars, including the funds that allowed the Met to continue its famous Saturday afternoon radio broadcasts after former sponsor Chevron Texaco withdrew its support. She also helped to choose Sony Classical president Peter Gelb as the successor to Volpe, who steps down in 2006.
Sills accepted the chairmanship of the Met just months after stepping down as chairman of Lincoln Center, a position she had held since 1994. Previously, she had served as the general director of New York City Opera, the company with which she sang for more than two decades in the 1950s, '60s, and '70s.