The incidence of the reproductive disease endometriosis was determined in a colony of rhesus monkeys chronically exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin) for a period of 4 years. Ten years after termination of dioxin treatment, the presence of endometriosis was documented by surgical laparoscopy and the severity of disease was assessed. The incidence of endometriosis was directly correlated with dioxin exposure and the severity of disease was dependent upon the dose administered (p < 0.001). Three of 7 animals exposed to 5 ppt dioxin (43%) and 5 of 7 animals exposed to 25 ppt dioxin (71%) had moderate to severe endometriosis. In contrast, the frequency of disease in the control group was 33%, similar to an overall prevalence of 30% in 304 rhesus monkeys housed at The Harlow Primate Center with no dioxin exposure. This 15-year study indicates that latent female reproductive abnormalities may be associated with dioxin exposure in the rhesus. Therefore, the effects of this toxin may be more diverse than previously recognized.