The clinical and serologic features and immune status of 39 homosexual men who had seroconversion to human immunodeficiency virus positivity were compared with 26 homosexual men who remained seronegative during a six-month period. An acute clinical illness occurred in 92.3% of seroconverted subjects and 40% of controls. The duration of illness was significantly greater in the seroconverters than the controls (10 + 4.4 days). A general practitioner was consulted by 87.2% of the seroconverters because of the illness, including 12.8% who were admitted to hospital, compared with 20% of controls. The most frequently reported symptoms in the seroconversion group were fever (76.9%); lethargy and malaise (66.7%); anorexia, sore throat, and myalgias (56.4% each); headaches and arthralgias (48.7% each); weight loss (46.2%); swollen glands (43.5%); retro-orbital pain (38.5%); and dehydration and nausea (30.8% each). Lymphadenopathy developed in 75% of seroconverters compared with 4% of controls. Changes in T-cell subsets were not found in controls, but the number of T4+ cells and the T4+/T8+ ratio decreased significantly in seroconverters.