Six children with the acquired immunodeficiency syndrome (AIDS) and 12 of their household contacts were investigated serologically for evidence of infection with human T-lymphotropic virus/lymphadenopathy-associated virus (HTLV-III/LAV), the presumed etiologic agent of AIDS. All six children had antibody against HTLV-III/LAV, as measured by enzyme-linked immunosorbent assay, in each specimen tested. Of the two mothers studied both were seropositive; one was diagnosed with and died from AIDS. Four of the remaining 10 household members were seropositive, including three adults in groups at high risk for the development of AIDS and one sibling who was younger than the child with AIDS. Among the seronegative household contacts were four foster mothers or grandmothers of the children with AIDS, three of whom had cared for the children since infancy. Household contact with children with AIDS may include persons in groups at high risk for AIDS who have been infected with HTLV-III/LAV. However, the negative findings in household contacts without risk factors for AIDS suggest that horizontal transmission of the virus within households by means other than sexual contact must be infrequent.