As President of the United States from 1909 to 1913, William Howard Taft's minimum body mass index was 42 kg/m(2). This article presents evidence that he suffered from obstructive sleep apnea, manifested by excessive daytime somnolence, snoring, systemic hypertension and, perhaps, cognitive and psychosocial impairment. As president, Taft's hypersomnolence was severe and obvious, but never prompted official discussion of his fitness to govern. Within 12 months of leaving office, Taft permanently lost over 60 pounds. His somnolence resolved. As Chief Justice of the United States from 1921 to 1930, he was not somnolent. President Taft's case illuminates historical puzzles of his performance as President, raises public awareness of sleep apnea, and informs discussions of presidential disability and the 25th Amendment to the Constitution of the United States.