Identification of laboratory tests that can help predict progression to acquired immunodeficiency syndrome (AIDS) in people infected with human immunodeficiency virus (HIV) is important for clinical management and counselling. We have assessed the usefulness of CD4 lymphocyte count, serum β2-microglobulin concentration, and the presence of p24 antigen as predictors of AIDS. We studied 214 homosexual and bisexual men with well-defined dates of HIV seroconversion. For each participant, we defined the baseline date as the earliest date before the development of AIDS on which the three laboratory tests were done. β2-microglobulin concentration at baseline was in all analyses an independent predictor of AIDS, even after stratification by baseline CD4 count, duration of HIV infection, or use of zidovudine before or at baseline. For example, among men with at least 0·5 x 109/l CD4 cells who were negative for p24 antigen, the risks of AIDS at 12 months and 24 months were 1% and 5%, respectively, for those whose β2-microglobulin concentrations were below 4·0 mg/l, compared with 17% and 27%, respectively for those with β2-microglobulin concentrations above that cut-off point (p<0·001). Among men with an estimated duration of infection of 5 years or less, β2-microglobulin concentration was the strongest independent predictor of AIDS. Measurement of serum β2-microglobulin adds important prognostic information to CD4 count in determining the risk of progression to AIDS in HIV-infected subjects, including those whose CD4 cell count has not vet fallen.