Serum samples from 24 patients with acquired immune deficiency syndrome (AIDS) and from 15 patients with an early or milder form of this disease ("suspected AIDS") were found to contain elevated levels of β2-microglobulin (β2M). Therefore, prospective studies of 40 asymptomatic homosexual men from New York City were undertaken to determine whether quantitation of β2M and other immunologic variables was useful in recognizing those in populations at high risk for this disease who have a high probability for experiencing symptoms consistent with AIDS. After 20 to 26 months of follow-up, two of those persons now have AIDS and four have suspected AIDS. All six of these persons had elevated serum β2M levels and other immunologic abnormalities when they entered this study. Of those tested, only one other man had an increased level of β2M; neither he nor any of the remaining 33 persons in this group developed AIDS.