We will prospectively study health-related and life-style characteristics of male homosexuals in a low incidence area for AIDS, i.e., Pittsburgh-Tri-state area. The goal is to uncover antecedents and risk factors of AIDS. We will study a cohort of 7,000-10,000 gay males for two and one half years each for development of AIDS and for the effect of introduction of the putative AIDS agent. This includes collection of sera for the AIDS Repository, medical-sexual-travel data, and screening for AIDS by questionnaire.