Women in Western nations are exposed to an "unnatural" high number of menstrual cycles and this has been claimed to favour the development of endometriosis. If so, the prevalence of the disease should be low in remote rural settings characterized by high fertility rate, frequent teen-age pregnancy and protracted breast-feeding. To verify this hypothesis, we investigated the prevalence of endometriosis among women referring to the District Hospital of Aber, Northern Uganda for gynecological complaints. Subjects were considered affected if they had a history of surgery for endometriosis or if they had a positive clinical or ultrasound examination. Overall, a total of 528 gynecological consultancies were performed during the one year study period. Endometriosis was recorded in only one case. The frequency of the disease in the whole cohort of referred cases was thus 0.2% (95% confidence intervals (CI): 0.01-0.9%). When focusing on non-pregnant women in their reproductive age, it was 0.3% (95% CI: 0.01-1.3%). When considering women complaining symptoms suggestive for endometriosis, it was 0.4% (95% CI: 0.02-1.9%). In conclusion, endometriosis is rare in a community characterized by high fertility rate, frequent teen-age pregnancy and protracted breast-feeding, supporting the idea that the reproductive pattern plays a crucial role in the pathogenesis of the disease.