The ovarian-peripheral gradients of various Δ4 and Δ5 steroids were determined for a patient with virilizing arrhenoblastoma. The high peripheral testosterone level accompanying this tumor results from increased precursor supply from both the Δ4 and Δ5 pathways, with the Δ5 pathway predominating, and from negligible aromatase activity. A review of 45 cases of androgen-producing ovarian tumors with measurement of peripheral venous testosterone, and of 24 cases with measurement of ovarian venous testosterone, and a comparison with findings in 159 patients with hirsutism of functional origin reveal the following: 1) An androgen-producing tumor must be ruled out when peripheral testosterone exceeds 2 ng/ml; 2) an ovarian venous testosterone level exceeding 20 ng/ml generally accompanies a tumor, particularly when the tumor is less than 5 cm in diameter; and 3) virtually all (98%) of the tumors reviewed were accompanied by virilization, regardless of the peripheral concentration of testosterone.