The long-term effects of seropositivity for human T-lymphotropic virus type III (HTLV-III) on T-lymphocyte subsets and health status were evaluated in longitudinal studies of 250 initially healthy homosexual men. The relative risk of having an inverted T-lymphocyte helper-to-suppressor ratio rose from 14.3-fold among short-term seropositive subjects (less than 19 months) to 46.9-fold among long-term seropositive subjects (greater than 29 months) in comparison with the risk among seronegative subjects. Overall, 91.7% of long-term seropositive men had inverted ratios, compared with 12.9% of seronegative men. None of the seropositive men who developed an inverted ratio later reestablished a normal ratio. Both decreased T-helper cell number and percentage (p = 0.003) and increased T-suppressor cell number and percentage (p = 0.03) were significantly correlated with duration of seropositivity. Among seropositive persons, lymphadenopathy was a highly significant short-term as well as long-term consequence, whereas diarrhea, oral thrush, and herpes zoster were correlated with long-term seropositivity. Overall, 50% of long-term seropositive men compared with 16% of seronegative men developed at least one of five clinical symptoms (p less than 0.003). We conclude that a high proportion of persons infected with HTLV-III will develop measurable immunologic and clinical abnormalities.