In the Houston Family Study, overall rates of infection for the three major outbreaks of influenza A from 1977 to 1981 were higher for subtype H3N2 than for H1N1. Rates in school children were almost identical, but rates of infection with H1N1 were lower in adults and in preschool children, especially those younger than two years of age. However, rates for the two subtypes were similar in young children within families that experienced influenza A infections. In the total population overall illness rates among infected persons were identical, and diagnoses were similar. Among 145 identified primary infections (1977-1982), more H3N2 infections resulted in systemic (febrile) illness or lower-respiratory-tract disease, but a detailed comparison of illness features of 54 primary infections in children aged two to five years showed no significant differences. Epidemiological differences appear to be more important than pathogenic potential in determining the community impact of these two subtypes of type A influenza virus.