Four assays for serum levels of cellular products of immune activation were examined as prognostic markers for AIDS in a prospective study of asymptomatic HIV-seropositive homosexual men. Baseline serum values of β2-microglobulin (β2M), neopterin, soluble CD8 (sCD8), and soluble interleukin-2 receptor (sIL-2R) for 185 men were examined univariately and multivariately as predictors of AIDS during 36 months of follow-up. Thirty-three cases of AIDS (18%) were diagnosed during the follow-up period. All four assays correlated highly with each other (r = 0.48–0.63), and all four were good univariate predictors of AIDS and comparable to CD4 lymphocyte count. β2M, neopterin, and sCD8 predicted AIDS independently of both CD4 count and HIV p24 antigen or p24 antibody in multivariate analysis. Within the range of CD4 count 200–499 × 106cells/I, an immune activation marker used in combination with an assay for p24 antigen identifies those at 3–6% risk of AIDS over 36 months (low risk on both assays) and those at 63–86% risk (high risk on both assays). These results can be used to guide physicians and patients making decisions about treating asymptomatic HIV infection with zidovudine in individuals with CD4 lymphocyte count of 200–499 × 106cells/l.