William Henry Harrison dies of pneumonia, becoming the first President of the United States to die in office, and setting the record for the briefest administration. Vice President John Tyler succeeds Harrison as President.

William Henry Harrison was the ninth president of the United States, serving from March 4 to April 4, 1841, the shortest presidency in U.S. history. He was also the first U.S. president to die in office, causing a brief constitutional crisis, since presidential succession was not then fully defined in the U.S. Constitution. Harrison was the last president born as a British subject in the Thirteen Colonies. He was a member of the Harrison family of Virginia, a son of Benjamin Harrison V, who was a U.S. Founding Father; he was also the grandfather of Benjamin Harrison, the 23rd U.S. president.

Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity of the condition is variable.

The president of the United States (POTUS) is the head of state and head of government of the United States. The president directs the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces.

John Tyler was the tenth president of the United States, serving from 1841 to 1845, after briefly holding office as the tenth vice president in 1841. He was elected vice president on the 1840 Whig ticket with President William Henry Harrison, succeeding to the presidency following Harrison's death 31 days after assuming office. Tyler was a stalwart supporter and advocate of states' rights, including regarding slavery, and he adopted nationalistic policies as president only when they did not infringe on the states' powers. His unexpected rise to the presidency posed a threat to the presidential ambitions of Henry Clay and other Whig politicians and left Tyler estranged from both of the nation's major political parties at the time.