Democratic presidential candidate Hillary Clinton on Sunday told reporters she was "feeling great" after becoming overheated at a September 11 memorial ceremony in New York City and leaving early.
Video showed Ms Clinton appearing to faint and stumble as she was led to a waiting van by her aides.
Ms Clinton made the remarks upon leaving her daughter Chelsea's home in Manhattan, where she was taken after departing the memorial service.
Ms Clinton left the event after about 90 minutes while it was still underway, Nick Merrill, a Ms Clinton spokesman, said. He gave no further details about her condition.
As the event began on Sunday at the site of the World Trade Center that was attacked by two hijacked airliners 15 years ago, there was patchy sunlight, with temperatures at about 80 degrees Fahrenheit (26.6 Celsius).
Democratic Representative Joe Crowley of New York, a supporter of Ms Clinton's who attended the event, told MSNBC television that it was "stifling" during the ceremony.
After some US media reported she had fallen ill, reporters travelling with the candidate did not get an immediate response from the campaign to questions about her whereabouts and her health.
About 90 minutes elapsed before the campaign issued a statement.
Republican presidential candidate Donald Trump repeatedly has raised questions about Clinton's health and whether she had the "stamina" to serve as president.
In December 2012, Ms Clinton, 68, suffered a concussion and shortly afterward developed a blood clot.
In a letter released by her doctor in July, Ms Clinton was described as being in "excellent health" and "fit to serve" in the White House.
Ms Clinton's speech at a campaign rally earlier this month in Cleveland was interrupted by a coughing spell. During the speech, she quipped, "Every time I think about Trump I get allergic". She then resumed her speech.
But the episode fueled speculation from conservative political quarters about her health. Trump supporters have been tweeting unsubstantiated theories regarding Ms Clinton's health under the hashtag #HillarysHealth