The MACS was initiated in 1983 to study the natural history of HIV infection among homosexual and bisexual men in the United States. The design of the MACS has been previously described,14 and only aspects pertinent to this study are presented herein. From April 2, 1984, to April 8, 1985, 4954 men, who were either seropositive or seronegative for HIV, were enrolled in the Baltimore, Md/Washington, DC, area, Chicago, Ill, Los Angeles, Calif, and Pittsburgh, Pa, with an additional 625 men enrolled from April 1, 1987, to February 25, 1991. Men with clinical AIDS or who were younger than 18 years were ineligible. At semiannual visits, the men returned to the clinics to provide specimens for laboratory analyses, undergo a physical examination, and complete self-administered data forms and an interviewer-administered questionnaire. At each visit, T-cell subset levels were measured in peripheral blood samples stained with monoclonal antibodies by means of a whole-blood lysing method and analyzed by means of 2-color flow cytometry15,16 and monoclonal antibodies specific for CD3, CD4, and CD8 lymphocytes. Absolute numbers of cells per microliter of blood were calculated using the complete blood cell count with an automated 10 000-cell differential. Because the MACS began before the advent of HIV RNA assays, HIV RNA data are limited. The HIV RNA data were used in this study for the baseline analyses and included only when available within 1 year of the visit.