During the 2007-08 influenza season, influenza activity peaked in mid-February in the United States and was associated with greater mortality and higher rates of hospitalization of children aged 0-4 years, compared with each of the previous three seasons. In the United States, influenza A (H1N1) was the predominant strain early in the season; influenza A (H3N2) viruses increased in circulation in January and predominated overall. While influenza A (H1N1), A (H3N2), and B viruses cocirculated worldwide, influenza A (H1N1) viruses were most commonly reported in Canada, Europe, and Africa, and influenza B viruses were predominant in most Asian countries. This report summarizes influenza activity in the United States and worldwide during the 2007-08 influenza season (September 30, 2007-May 17, 2008).