In an attempt to determine the rate of transmission of infection from human immunodeficiency virus type 1 (HIV-1) antibody-positive women to their offspring and to describe the short-term outcome of perinatal infection, we enrolled 62 infants in a prospective cohort study during a 30-month period and followed them up for an additional 6 months. The clinical, immunologic, and serologic status of the children was assessed prospectively. Fourteen subjects were symptomatic: 3 had acquired immunodeficiency syndrome, 5 had signs and symptoms that were compatible with HIV-1 infection (Centers for Disease Control, Atlanta, Ga, class P2A), and 6 had ill-defined symptoms that could not be definitely attributed to HIV. Our data indicated that the maximum rate of vertical transmission of HIV-1 infection in New Haven, Conn, was less than 30%, and the rate of HIV-1-associated disease occurring during the first 3 years of life was 16%. The mean and median time to loss of maternal antibody, as detected by Western blot in seroreverters, was approximately 7 months, and the half-life of passive antibody was 38 days. A continued close follow-up of children in the cohort studied, and others like it, is critical to learn the full range of outcomes of HIV infection in the pediatric population.