The presence of a virilizing ovarian hilum cell tumours was suspected on account of a rise of plasma testosterone level above 3.5 ng/ml in one patient and a rise of plasma androstenedione level above 3.0 ng/ml in another. The diagnosis was made by selective catheterization of the ovarian and adrenal veins, which showed a high androgen concentration gradient in the vein of the ovary affected as compared with that observed in the adrenal and peripheral veins, and was confirmed by histological examination after ovariectomy. These two cases are contrasted with two other cases with high testosterone plasma levels but without gradient on selective catheterization. One of the patients had abnormal hepatic metabolism (porto-caval shunt) and the other had taken high doses of androgens.