The New York Times on Friday reported the 58-year-old Clinton suffered a heart attack.
Clinton is now at the Columbia-Presbyterian hospital in New York which is known for its cardiology department.
CNN television said that surgery would be carried out on Saturday and that Clinton had gone to hospital suffering from chest pains and fatigue.
Fox News television, quoting a Clinton friend, said the ex-president went to the hospital after a heart blockage was found during tests.
A Columbia-Prebyterian hospital spokesman told AFP: "No one can comment at this point."
Clinton has spent recent months promoting his best-selling autobiography "My Life". He also keeps up a punishing schedule of international speaking engagements while his wife Hillary pursues her political career in the US senate.
Hillary Clinton was at an engagement in New York state on Friday.