SummaryWe studied human immunodeficiency virus type 1 (HIV-1) infection incidence over time in a 16-center cohort of hemophiliacs in the United States and Europe and estimated the most likely date of seroconversion for all seropositive subjects. Five U.S. centers enrolled subjects independent of HIV-1 status, whereas 11 centers preferentially included seropositive subjects. We obtained unbiased estimates of HIV-1 infection incidence rates from the five centers and estimated dates of seroconversion from the distribution seen among seropositives from all centers. In the five-center cohort, infection incidence began in 1978, peaked in October 1982 at 22 infections per 100 person-years at risk, and declined to 4 per 100 person-years by July 1984. Few infections occurred after 1987, and by that time, 50% of the cohort had become infected. Median seroconversion dates for subgroups of all seropositives ranged from July 1980 to December 1983, depending on the dose and type of factor concentrate. Median dates in Europe ranged from September 1981 to March 1983 and reflected the use of products manufactured from American plasma. Infection incidence apparently peaked about the same time that public health interventions were introduced to reduce transmission. These interventions, including heat treatment of factor concentrates and deferral of high-risk donors, have prevented HIV-1 infection from becoming endemic among younger birth cohorts of persons with hemophilia.