The discovery of human T-lymphotropic virus type III/lymphadenopathy-associated virus (HTLV-III/LAV) has opened a window to the understanding of the spectrum of the acquired immunodeficiency syndrome (AIDS) and related clinical syndromes. Analysis of risk factors for seropositivity has shown that HTLV-III is transmitted most efficiently via routes that involve close personal contact or parenteral exposure. Longitudinal studies have shown that HTLV-III infection has a long latent period. The prevalence of AIDS in different geographic areas and among different risk groups appears to depend in part on duration of exposure. Co-factors for AIDS outcome such as manner and route of exposure, underlying immune status, and host susceptibility are also likely to play a role in risk.