The incidence of invasive Haemophilus influenzae infections was studied among all residents of Rochester, Minnesota, under 20 years of age during the period 1975-1983. The incidence in children under 5 years of age was 115 and 64 cases per 100,000 children per year for all invasive cases and for meningitis, respectively. These rates are among the highest reported for an essentially all-Caucasian population in the United States, while the ratio of meningitis to non-meningitis cases (1.1:1) is among the lowest. This suggests possible under-ascertainment of non-meningitis cases in previous US studies of Haemophilus influenzae infections.