We studied the incidence of Hemophilus influenzae meningitis at the Columbus Children's Hospital (CCH) during the following intervals: Period A (1942 to 1950), Period B (1951 to 1959), and Period C (1960 to 1968). Other investigators have reported an increase in the incidence of influenzal meningitis, but there has been no predictable relationship to the number of hospial admissions. Thre was a 399% increase in the number of patients with H. influenzae meningitis at the CCH from Period A to Period C, roughly proportional to the rise in total hospial admissions during he same time. Children under 5 years of age constitute the major population at risk. Incidence stated in terms of population at risk appears to be a reliable method of comparing experience in specific geographic areas during specific periods of time. The experience at the CCH, using this method, indicates a significant increase in this disease without comparable increases in meningitis caused by other bacteria.