Two hundred twenty-nine patients in Haiti with the acquired immunodeficiency syndrome were studied between 1979 and 1984. The clinical spectrum of the syndrome in Haitians was similar in most aspects to that in patients with the disease in the United States. However, in contrast to findings in the United States, accepted risk factors (bisexuality, blood transfusions, intravenous drug abuse) were identified in only 43% of Haitian patients. Patients in Haiti with and without these risk factors were similar to each other but differed from age- and sex-matched siblings and friends in the number of heterosexual contacts and receipt of intramuscular injections. These latter activities were commoner in patients than in their siblings and friends, and represent potential modes of transmission of infection with the human T-lymphotropic virus type III.