Household members of 46 patients with confirmed acquired immunodeficiency syndrome (AIDS) and 43 human T-cell lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV)-seronegative controls from Kinshasa, Zaire, were identified and sought for serologic testing for evidence of HTLV-III/LAV infection. Twenty (9.8%) of 204 case-household members and three (1.9%) of 155 control-household members were HTLV-III/LAV seropositive (relative risk = 5.1; 95% confidence interval, 1.7 to 15.2). Eleven (61.1%) of 18 spouses of patients with AIDS were HTLV-III/LAV seropositive, compared with one (3.7%) of 27 control spouses (relative risk = 16.5; 95% confidence interval, 3.7 to 75.0). Except for spouses, the rate of HTLV-III/LAV seropositivity did not differ significantly between case and control households. Furthermore, for adults in case households who were not spouses, the number seropositive for HTLV-III/LAV was identical to that predicted from sex- and age-specific HTLV-III/LAV seroprevalence rates. These data from Zaire confirm the results of US and European studies of household contacts of infected hemophiliacs and pediatric patients with AIDS.