Twenty-seven cases of endometriosis were found in 94 female cynomolgus monkeys retired from breeding. The ages of the affected monkeys (28.7% of all the monkeys) ranged from 11 years to 23 years of age with an average of 15.3 years. There was no significant difference in the incidence of endometriosis between groups which had or had not undergone a cesarean section. Time after the last pregnancy was significantly longer in the affected monkeys than in the non-affected ones. These cases were histopathologically classified into four types: 1) pelvic endometriosis with formation of endometrial cysts (51.8% of the affected monkeys), 2) pelvic endometriosis invading other organs without formation of endometrial cysts (3.7%), 3) uterine adenomyosis (22.2%), 4) combined pelvic endometriosis and uterine adenomyosis (22.2%).