Influenza activity in the United States increased substantially during mid-December 1999 and appears to have peaked during the weeks ending December 25 (week 51) through January 15 (week 2). Predominant viruses isolated this season have been influenza type A(H3N2) viruses, antigenically similar to the viruses that have predominated since the 1997-98 influenza season and were well matched to this season's vaccine. This report summarizes influenza activity in the United States during October 3,1999-February 26, 2000, and compares the current season with the five previous seasons.