During the 2004-05 influenza season, influenza A (H1),* A (H3N2), and B viruses cocirculated worldwide, and influenza A (H3N2) viruses predominated. In addition, several Asian countries continued to report widespread outbreaks of avian influenza A (H5N1) among poultry; in Vietnam, Thailand, and Cambodia, these outbreaks were associated with severe illnesses and deaths among humans. In the United States, the 2004-05 influenza season peaked in February, was moderate, and was associated predominantly with influenza A (H3N2) viruses. This report summarizes influenza activity in the United States and worldwide during the 2004-05 influenza season.