OBJECTIVE To evaluate the safety and efficacy of an antiprogesterone (mifepristone, RU486; Roussel-Uclaf, Romaineville, France) on endometriosis. METHODS An open, prospective clinical trial. METHODS The clinical practice of an academic faculty. METHODS Nine women with endometriosis were studied. METHODS RU486 (50 mg/d) was administered for 6 months. METHODS Daily symptom inventories and urinary steroid metabolites were assessed before, during, and after treatment. Blood for hormone analysis was obtained weekly for 4 weeks and monthly thereafter. The extent of endometriosis, bone mineral density, circadian rhythm of cortisol, and LH pulsatility were determined before and after treatment. Safety laboratory measurements were made before and at 1, 2, and 6 months of treatment. RESULTS Pelvic pain and uterine cramping improved in all patients. Endometriosis regressed by 55%. All patients exhibited endocrine features of anovulatory amenorrhea without hypoestrogenism. A rise in serum LH and T levels was observed during the first month of treatment and one patient developed an elevation of liver transaminases during the last month of treatment. All other measurements were unchanged. CONCLUSIONS RU486 appears to be effective in improving the symptoms and causing regression of endometriosis in the absence of significant side effects.