The Multicenter AIDS Cohort Study was designed to 1) elucidate the natural history of the infection causing acquired immunodeficiency syndrome (AIDS), 2) identify risk factors for occurrence and clinical expression of the infection, and 3) establish a repository of biologic specimens for future study. A variety of recruitment techniques, including special assurance of confidentiality, were used to enroll participants. Nearly 5,000 homosexual men volunteered for semiannual interview, physical examination, and laboratory testing in four metropolitan areas. A significant majority of these men in each center (69–83%) reported having 50 or more lifetime sexual partners, and over 80% had engaged in receptive anal intercourse with at least some of their partners in the previous two years. By the time of the participants' initial evaluation (April 1984-April 1985), infection with the human immunodeficiency virus (HIV) had occurred in higher proportions of men in Los Angeles (51%) and Chicago (43%) than in Baltimore/Washington, DC (31%) and Pittsburgh (21%), presumably as a result of the higher number of partners and proportion with whom these men had engaged in high-risk practices (e.g., receptive anal intercourse). Follow-up evaluations are underway in this comprehensive longitudinal investigation of HIV infection.