The autopsy records between 1980 and 1995 of 399 female rhesus monkeys (Macaca mulatta) at the Wisconsin Regional Primate Research Center were examined. Spontaneous endometriosis was found in 81 (20%) of the animals. The mean (+/- SD) ages at death for animals with and without endometriosis were 20.7 +/- 5.5 (range 10-35) and 13.4 +/- 7.7 (range 4-37) years respectively. Many of the animals had been exposed to experimental procedures, including laparoscopies, hysterotomies and oestradiol implants, and these were examined as possible risk factors for endometriosis. Of the 81 affected animals, 62 were matched to unaffected controls for age at death (to within 1 year) and year of death (to within 2 years) and the effect of various factors on the development of endometriosis was determined using conditional logistic regression. Exposure to three or more oestradiol implants or one or more hysterotomies were both significant risk factors, with estimated relative risks of 9.7 (95% confidence interval 2.5-37.2) and 5.8 (95% confidence interval 1.6-20.2) respectively. Animals that had been exposed to one or more laparoscopies showed no increased risk for developing endometriosis. These findings provide insight into the aetiology of the disease in women. ?2P51 4RR00167