A prospective study to evaluate the efficacy of danazol in the treatment of endometriosis was initiated in 1976. One hundred patients with endometriosis, as demonstrated by laparoscopy, were entered into the study. The mean length of patient follow-up was 49 months. All patients were treated with 800 mg of danazol per day. The mean duration of danazol therapy was 17.3 weeks. Eighty-nine percent of the patients reported symptomatic improvement, and 94% were improved, as demonstrated by repeat laparoscopy or laparatomy. After completing a course of danazol therapy, 57% of the patients underwent conservative laparotomy for fertility enhancement, pain control, or ovarian masses. Two patients underwent total abdominal hysterectomy-bilateral salpingo-oophorectomy for advanced disease poorly responsive to danazol therapy. In those patients desiring fertility (56), there were 38 pregnancies in 26 women, for an overall fertility rate of 46%. The overall recurrence rate was 33%, as determined by symptoms and physical findings. Significant side effects from danazol were reported by 85% of the patients. The major side effects were weight gain, edema, decrease in breast size, oily skin, hirsutism, and deepening of the voice. Only one patient discontinued danazol therapy due to side effects. Clinical tissues concerning the use of danazol with or without surgery and the miscarriage rate following danazol therapy are discussed.