As of 2014, eight U.S. presidents have died while in office: William Henry Harrison, Zachary Taylor, Abraham Lincoln, James A. Garfield, William McKinley, Warren G. Harding, Franklin D. Roosevelt and John F. Kennedy. Four of these men (Lincoln, Garfield, McKinley and Kennedy) were assassinated, and the other four (Harrison, Taylor, Harding and Roosevelt) died of illness or other natural cause while serving as president of the United States. Though only four presidents have been confirmed to have been assassinated, at least two of the presidents who died of natural causes while in office, Taylor and Harding, were the subject of rumors of covert assassination by poisoning at the time of their deaths.
In the case of Zachary Taylor, forensic evidence gathered in the 1990s ruled out the possibility of poisoning, though the rumors of President Harding's demise were never officially cleared up. His wife refused to allow an autopsy and had hired a private investigator to look into her husband's known infidelities, leading some to speculate that Mrs. Harding killed her husband. However, doctors at the time agreed that President Harding, who suffered from high blood pressure, likely died of a heart attack. In President Harrison's case, his death while in office came only one month after his inauguration speech, making him the shortest serving president in history.