Epidemics of influenza of the past 10 years were reviewed by integrating records from Morbidity and Mortality Weekly Reports of the United States and Infectious Agents Surveillance Reports in Japan, as well as data of antigenic drift of epidemic strains. Nucleotide-sequencing analyses of field isolates have shown characteristic amino acid substitutions in their hemagglutinin molecules, and phylogenetic analyses have indicated evolutionary relationships among the viruses. Integration of information obtained from both epidemics and molecular analyses would help our understanding of antigenic drift and evolution of influenza viruses.