Attempts to predict the course of the epidemic of acquired immunodeficiency syndrome (AIDS) have been hampered by the lack of an objective, practical way to estimate the prevalence of infection with the human immunodeficiency virus (HIV) in the general population. Testing for the prevalence of HIV infection in women should be a sensitive means to track the epidemic and to study the potential for perinatal transmission. Antibodies in maternal blood are contained in neonatal blood specimens routinely collected on absorbent paper for other purposes, such as screening for phenylketonuria; we therefore tested for HIV antibody in these specimens. Analysis of batches of individually blinded specimens from selected hospitals protected the anonymity of the mothers and babies and was cost efficient. Using the newborn's blood as an indicator of the mother's serologic status, we concluded that 1 of every 476 women (2.1 per 1000) giving birth in Massachusetts was positive for HIV antibody by immunofluorescence assay or enzyme-linked immunosorbent assay, both confirmed by immunoblot (Western blot) testing. The prevalence of HIV infection varied according to the type and location of the maternity hospitals; rates of seropositivity were highest in inner-city hospitals (8.0 per 1000), lower in mixed urban and suburban hospitals (2.5 per 1000), and lowest in suburban and rural hospitals (0.9 per 1000). This method is useful for collecting data needed to plan and evaluate prevention strategies and to predict the health care resources that will be needed to care for women and children who contract AIDS. Because other states have newborn screening programs similar to the Massachusetts program, this approach can be used for national surveillance of AIDS in women.