One hundred and fourteen household contacts of 23 patients with AIDS or suspected AIDS were examined for evidence of infection with HIV. In total 12 contacts had antibodies to the virus by a competitive enzyme-linked immunosorbent assay (ELISA), confirmed by Western blot. Ten of these seropositive contacts were the sexual partners of the index cases. Overall 71% of sexual partners (10:14) were seropositive. By contrast only 2:100 of the non-sexual contacts had evidence of infection. These were the daughter of an index case and her 2-year-old son who were seropositive but had their own independent risk factors. The data presented here support the hypothesis that a major route of transmission of HIV in an African population is through heterosexual contact and furthermore suggest that transmission by other means such as intimate non-sexual contact, insect vectors, or the use of non-sterile needles is much less important.