Clinical, immunological, microbiological, virological, and lifestyle parameters were followed in 200 homosexual men living in Finland. The subjects were seen at 3- to 6-month intervals starting in summer 1983. Human T-cell lymphotropic virus III (HTLV-III) antibodies detected by enzyme immunoassay and confirmed by Western blotting were seen in 18 (9%) of the cases. Initially two cases had acquired immunodeficiency syndrome, three had acquired immunodeficiency syndrome-related complex, three had lymphadenopathy syndrome, and ten were asymptomatic. During the follow-up two asymptomatic cases developed lymphadenopathy syndrome and three developed enlarged lymph nodes. Immunological studies revealed decreased T-helper cell values and/or T-helper/suppressor ratios in all clinical categories, the findings being more severe and progressive in the symptomatic cases. The finding most clearly distinguishing HTLV-III antibody positive cases from the antibody negative ones was a decreased response to a specific recall antigen, purified protein derivative of tuberculoprotein. Responses to mitogens phytohemagglutinin and pokeweed mitogen were also decreased, but to a lesser extent. The primary immunological defect associated with helper T-cell infection by HTLV-III thus seems to be loss of antigen specific immune responses. It is suggested that this is the result of previous antigenic stimulation of HTLV-III infected T-helper cells and that the cellular destruction is associated with initial mitotic activity.