The striking similarities between simian immunodeficiency virus (SIV)-induced disease in macaque monkeys and HIV-induced disease in humans make the SIV-induced macaque monkey an extraordinarily important model for the study of AIDS. The most significant difference between these lentivirus-induced syndromes is the more rapid progression of disease in SIV-infected monkeys. The immunologic and pathologic manifestations of SIV infections in rhesus monkeys are described.