Examined the effects of HIV infection and prenatal drug exposure on infant neurodevelopmental functioning. Three groups of infants were compared: HIV-infected infants, seroreverters, and a comparison group who were prenatally exposed to drugs, but not HIV. Two thirds of the HIV-infected and seroreverter infants were prenatally drug-exposed. Infants (ages 4-30 months) were administered the Bayley Scales of Infant Development. Children who were both HIV-infected and prenatally drug exposed performed significantly lower on both the mental and psychomotor scales of the Bayley. Drug exposure and neurological dysfunction were associated with mental development, whereas neurological dysfunction, drug exposure, and HIV status were associated with psychomotor development.