Danazol and gestrinone are effective drugs in the treatment of endometriosis. Their mechanism of action remains uncertain, but may be related to their androgenic activity. The authors examined the effect of danazol on human endometrial cells cultured in vitro, its two major metabolites, ethisterone and 2 hydroxymethyl ethisterone, gestrinone, and testosterone (T) at 1X and 10X expected plasma concentrations. Danazol and T suppressed growth by 20.8 and 25.0% (P less than 0.01), respectively, at the lower dose, and by 26.9 and 35.5% (P less than 0.01), respectively, at the 10-fold higher dose. No significant suppression of growth occurred with gestrinone, ethisterone, or 2 hydroxymethyl ethisterone. The results provide further evidence that danazol and T (but not gestrinone) may act by a direct effect on endometrial tissue.