Although danazol has been reported to inhibit endometrial cell growth in vitro, it is difficult to accept that this is through direct inhibition of danazol on endometrial cells. This is because local danazol therapy improves endometriotic signs and symptoms without any disturbance in ovulation or the menstrual cycle. We have re-evaluated the effects of danazol on human endometrial cells by using normal human endometrial stromal cells and two cell lines derived from highly-differentiated endometrial adenocarcinomas. Danazol is difficult to dissolve in aqueous solutions, and undissolved danazol significantly inhibited endometrial cell growth even at less than 100 ng/ml. At 500 ng/ml of fully dissolved danazol, which is the therapeutic dose, danazol solution inhibited leukemic cell growth but not endometrial cell growth. Therefore, the action of danazol may be not an inhibitory effect but rather a regulatory function of endometrial cell growth.