ObjectiveTo estimate the seroincidence of HIV-1 infection in the general adult population of Kigali, Rwanda.DesignRepeated standarized cross-sectional studies.SettingTwo urban prenatal clinics.PatientsA total of 4486 consecutive pregnant women consulting in 1989 and 1990.Main outcome measuresPrevalence of HIV-1 antibodies.ResultsHIV seropositivity increased by 3–5% annually over this period, indicating that it has not reached a plateau in this sentinel population. The percentage infection rates, calculated using two complementary methods, were 26.2–30.7% in 1990. Extrapolating these results to the general population of Kigali, we estimate that 2300–3800 new infections in young women and 3600–6100 new infections in young men occur annually among the total population of 350000 in Kigali.ConclusionsA new HIV infection occurred in an adult every 50–90 min, on average, in Kigali during 1989–1990, while every 6–7 h a baby with maternally acquired HIV infection was born. Our HIV surveillance system, which is based on prenatal sentinel posts, is a useful tool for monitoring the progression of the HIV epidemic in Kigali.