A 22-year-old male with bilateral testicular tumors and the 21-hydroxylase variety of congenital adrenal hyperplasia (CAH) was studied. Preoperatively, on his usual glucocorticoid regimen, his urinary pregnanetriol excretion was increased (8.0–23.5 mg/day), serum LH and FSH were normal to increased (14.3/28.7 mlU/ml and 13.2– 19.5 mlU/ml, respectively) and testosterone (T) was normal to decreased (176–600 ng/dl). At surgery, testicular vein concentrations of 17-α-hydroxyprogesterone (17-OHP) and androstenedione (Δ) were increased (30.1 /ig/dl and 38.3 /g/dl, respectively) while T was decreased (1,503 ng/dl); a positive peripheral vein—testicular vein gradient was not seen for these steroids. Following injection of 10 U of crystalline ACTH into the testicular artery, testicular vein concentrations of 17-OHP, Δ and T increased to 729 μg/dl, 2,390 μig/dl and 9,660 ng/dl, respectively. Microscopic examination of the testes revealed multinodular tumors composed of polygonal or rounded eosinophilic cells, arranged in cords, nests and clusters. The tumors extended from the hilus and compressed the adjacent testicular tissue. Electron microscopic examination of the tumors showed features common to steroid-secreting tissues, with abundant smooth endoplasmic reticulum in close proximity to mitochondria which was moderate in number. The adjacent testicular tissue was composed of immature tubules with normally developed Leydig cells in the interstitial tissues. From these data and a survey of previous works, it was postulated that these tumors were dependent upon ACTH for growth and steroid secretion. In view of the high serum LH concentration seen in association with incomplete suppression of adrenal steroid secretion in this study and the association of evidence of gonadotropin secretion with testicular tumors in other CAH patients, LH may also have contributed to the growth of these tumors.