This paper reviews the major features of a simian model of acquired immunodeficiency ('SAIDS'), SAIDS occurs endemically in colonies of macaque monkeys in the United States and resembles AIDS in humans in overall clinical manifestations, pathology, and immune deficiency. An infectious type D retrovirus, related to but distinct from the Mason-Pfizer monkey virus, has been identified as the primary cause of SAIDS. The relevance of these findings for human AIDS is discussed.