We describe endometriosis in an aged rhesus macaque. There was a large mass and a related paraovarian cyst, typical of the disease. Endometriosis is a common finding in nonhuman primate. In this report, we also review the pathophysiology of the disease and summarize the historical and more recent relevant literature. Given the frequency of endometriosis in the rhesus monkey and the long-life spans (15-30 years) of nonhuman primates in captivity, endometriosis should be suspected in animals displaying the earliest signs of the disease: anorexia, dysmenorrhea, menorrhagia, irregular menstrual cycles, or infertility. Despite recent advances in the diagnosis and therapeutic strategies for endometriosis in women, the disease remains a significant cause of morbidity and ultimately, a cause of mortality, in the older nonhuman primate.