One hundred and sixty-seven homosexual men in Los Angeles characterized by HIV antibody, T-cell numbers, titres to cytomegalovirus (CMV), and specific sexual practices were followed for two years for immune changes and for more than three years for development of clinical AIDS. Thirty-five per cent had antibody to HIV at baseline. The mean level of T-helper (Th) cells was significantly lower and of T-suppressor (Ts) cells significantly higher in HIV seropositives than in seronegatives. The annualized incidence of HIV seroconversion was 7%. Eight men developed AIDS, an attack rate of 14% in those with HIV antibody at baseline. A number of observations were made: (1) T-cell alterations, except a transient elevation in Ts cells, were unusual in the absence of HIV antibody; (2) a seropositive man with a T-cell alteration was significantly less likely to revert to ‘within normal limits’ than was a seronegative man; (3) a steady decline in the number of Th cells preceded onset of clinical AIDS; (4) the number of Ts cells remained higher in men subsequently developing AIDS than in other seropositive men; (5) clinical AIDS occurred only in men with HIV antibody whose CMV antibody levels were above the median for the group (1:1600); and (6) the attack rate for clinical AIDS was 50% in men with HIV antibody and elevated CMV who at baseline had either: (a) fewer than 325 Th cells/cc, or (b) whose Th/Ts ratio was below 0.8 (but whose levels of Th and Ts cells were within normal limits). This information should be helpful to clinicians caring for homosexual and bisexual men and for identifying suitable populations for future early intervention efforts.