We treated 36 women with laparoscopically proven endometriosis with danazol 600 mg/d (n = 17) or gestrinone 5.0 mg/wk (n = 19) for 6 months. Endometrial samples were obtained before and at 3 and 6 months of treatment and were studied by light, scanning, and transmission electron microscopy. At 3 months of treatment, the endometria of the danazol-treated patients were more atrophic than those of the women who received gestrinone. Some cell organelle involution was evident in patients of both treatment groups. After 6 months of treatment, marked endometrial atrophy was observed in all the patients, including those in whom spotting had occurred. The ultrastructural investigation demonstrated complete involution of the cytoplasmic organelles with cytoplasmic collapse in glandular cells of patients treated with danazol, whereas in the gestrinone group degeneration phenomena were observed in both nucleus and cytoplasm. Irregular secretory transformation was seen in the endometria of patients in both groups. Long-term treatment with danazol caused endometrial atrophy similar to that induced by gestrinone, but it appeared earlier; thus, the former drug seems preferable in short-term treatment.