Rhesus macaques (Macaca mulatta) with endometriosis were identified using reproductive histories, serum levels CA-125, pelvic ultrasonography, laparoscopy, and histopathology. All animals were evaluated from a large breeding colony and had a history of infertility and/ or spontaneous abortions. Laparoscopy and ultrasonography were performed on 40 macaques: 27 macaques from the breeding colony with elevated CA-125 levels, ten macaques from the breeding colony with normal or low serum CA-125 levels, and three macaques from another facility with previously diagnosed spontaneous endometriosis. Clinical endometriosis was diagnosed by laparoscopy in 16/37 (43%) macaques from the breeding colony and was confirmed by histologic examination in all animals biopsied. The disease was classified as minimal (40%), mild (25%), moderate (10%), or severe (25%). The most common sites of endometriosis were the serosal surface of the uterus (75%) and the posterior cul-de-sac (75%). In this study, CA-125 levels were useful in identifying animals from the breeding colony with endometriosis. The rhesus macaque provides a valuable animal model to study endometriosis and potentially to assess efficacy of therapeutic agents for this disease condition.