Clinton aides said doctors believed she had a stomach virus. She fell ill about five minutes into her lunchtime speech, said Erie County Democratic Chairman Len Lenihan, who was at the Women's TAP fund-raiser at a private club.
Clinton left the podium and continued speaking from a chair, then said she needed a break and left the room, Lenihan said. She returned to the podium a short time later but fainted before resuming her speech.
"She was sort of brought down gracefully," he said.
The room was cleared and Clinton was checked by a doctor who attended the event.
Clinton, 57, was smiling when she walked out of the club, general manager Vincent Tracy said. "I saw her walk out the door by herself, she smiled and said 'thank you'," he said.
Later Monday, she delivered a speech about health care at Canisius College. She was introduced by former Rep. John LaFalce, who told the crowd that Clinton went against doctor's advice to continue her daily schedule because she was committed to talking about health care.
Clinton was to return to Washington after the speech.
During an earlier editorial board meeting Monday at The Buffalo News, Clinton complained about suffering from a flu-like bug that affected several of her staff members during a weekend retreat in Westchester County, where she lives.
In September, Clinton's 58-year-old husband, former President Bill Clinton, underwent quadruple bypass surgery.
The former first lady has said she plans to run for re-election in 2006.