This study was done to gain insight into the clinical spectrum and immunologic disturbances resulting from infection with the human T-cell lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV) in children. Serum antibody to p41 antigen of HTLV-III and/or direct evidence of HTLV-III in lymphocytes was considered indicative of HTLV-III/LAV infection. In 36 children with HTLV-III/LAV infection, a wide clinical spectrum was noted, ranging from asymptomatic (seven children) to symptomatic, the latter including 14 children with the acquired immunodeficiency syndrome. Microcephaly was noted in five symptomatic infants. Immunologic dysfunction was noted in all symptomatic and in two asymptomatic children. Panhypogammaglobulinemia was noted in one child. Two asymptomatic children who were HTLV-III antibody negative had virologic evidence of HTLV-III infection. All of 20 mothers who were studied were HTLV-III antibody positive and had immunologic abnormalities but only nine were symptomatic, indicating that apparently healthy women may transmit HTLV-III/LAV infection to their offspring.