To examine the relationship between human immunodeficiency virus (HIV) seropositivity and T lymphocyte subsets in a clinically well population, we assayed HIV antibody and analyzed T lymphocyte subsets in 30 people at increased risk for acquired immunodeficiency syndrome (AIDS) who were clinically well. Seventy-six percent of the HIV-seropositive individuals had abnormally low numbers of T helper lymphocytes, and HIV seropositivity was strongly correlated with an abnormally low number of T helper cells (p less than 0.00002). Among these clinically well subjects at increased risk for AIDS, HIV-sero-positive individuals had a significant decrease in mean T helper lymphocytes and mean T helper:T suppressor ratios as compared to those who were seronegative (483 cells/mm3 vs 915 cells/mm3, p less than 0.002; and 0.80 vs 1.7, p less than 0.002, respectively). Because of the strong correlation of HIV seropositivity and abnormally low numbers of T helper lymphocytes in this asymptomatic population, these findings suggest that asymptomatic seropositive individuals should be followed closely for development of AIDS-related disease and should be considered for future antiviral therapy when it becomes available.