CD4 and CD8 T lymphocytes of three groups of men infected by the human immunodeficiency virus (HIV) were examined twice yearly for 36 months to elucidate the temporal trends in T lymphocytes during infection. The three groups were 37 HIV seroconverters, 304 prevalent HIV seropositives remaining free of the acquired immunodeficiency syndrome (AIDS), and 69 men who developed AIDS during observation. Six months before seroconversion, CD4 levels were similar among HIV seroconverters and 356 seronegative controls. Within 18 months of seroconversion, mean CD4 levels fell to the level of the prevalent seropositives at study entry. From there, the rate of decline slowed. CD8 lymphocyte counts rose dramatically at seroconversion. Among AIDS-free prevalent seropositives, CD4 levels fell steadily over 36 months of observation. By contrast, CD8 cell levels rose slowly. Among men who developed AIDS, mean CD4 levels fell more rapidly again during the 18 months prior to diagnosis. CD8 cell levels remained elevated until 6–12 months before diagnosis, when they began to fall.