The present study reports the case of a 25-yr-old man with congenital adrenal hyperplasia due to 21-hydroxylase deficiency in whom bilateral testicular tumors did not regress after suppressive treatment with dexamethasone. Catheterization of left testicular and adrenal veins confirmed the enzyme deficiency in the gonadal lesions. The presence of specific 11 beta-hydroxylated steroids (11 beta-hydroxyandrostenedione, 21-deoxycortisol, and 21-deoxycorticosterone) in the gonadal vein demonstrated the adrenal nature of the testicular tumor. In addition, catheterization allowed further study of the secretion of mineralocorticoids and androgens in the adrenal venous effluent. Plasma levels of deoxycorticosterone were increased in the peripheral vein and decreased in the adrenal vein, confirming the conversion of progesterone by peripheral 21-hydroxylase activity. Plasma levels of delta 5-3 beta-hydroxysteroids, particularly dehydroepiandrosterone and its sulfate, were very low, suggesting a sustained stimulation of 3 beta-hydroxysteroid dehydrogenase activity. This study documents that in patients with congenital adrenal hyperplasia and bilateral testicular tumors, catheterization of a gonadal vein measuring specific 11 beta-hydroxylated steroids confirms the adrenal nature of the gonadal lesions.