Substantial advances have already been made in the understanding of acquired immunodeficiency syndrome (AIDS). The major issues for AIDS research during the next few years must be practical ones: the development of a safe, effective vaccine for individuals not yet infected with the causative virus and the development of drug therapies for those already infected. Suitable animal models will be needed for studies designed to achieve these goals. Areas of investigation in animal models can be divided into four categories on the basis of increasing direct relevance to AIDS in humans: (1) retroviruses that have no obvious, close relation to human immunodeficiency virus (HIV) but can induce chronic diseases with manifestations that include immunologic abnormalities; (2) ungulate lentiviruses; (3) HIV-related viruses of Old World primates; and (4) HIV infection of chimpanzees. It is hoped that important research developments in experimental models can be quickly extrapolated to human AIDS.