Objective A long-term, follow-up study comparing mild and severe forms of endometriosis and their fecundability, on 28 women diagnosed with endometriosis in adolescence. Methodology Twenty-eight patients were identified from a prospective cohort of 52 adolescents (ages 12 to 18 years) with operative diagnosis of endometriosis between July 1993 and December 1995. All patients presented with chronic pelvic pain unresponsive to conservative medical management. Diagnosis of pregnancy was made by sonographic identification of intrauterine pregnancy, positive serum human chorionic gonadotropin or pathological confirmation of products of conception. Patients were categorized as fertile or sub-fertile by having > 12 months of unprotected intercourse without conception. Follow-up was done for 8.6 years. Results Staging of endometriosis was performed according to the American Society for Reproductive Medicine standards. Stage I = 14.3%; Stage II = 39.3%; Stage III = 42.8%; Stage IV = 3.6%. Fecundability rates in each stage were statistically significant: Stage I (75%), Stage II (55%), Stage III (25%), Stage IV (0%) (p < .05). Rates of spontaneous abortion were not statistically significant. Conclusion In our cohort, even at the earliest point in the natural life cycle of endometriosis there is an inverse relationship between stage of disease at diagnosis and fecundability.