Enlargement of the right breast, axillary hair, and acceleration of linear growth rate were first noted at 8 years of age in an otherwise healthy male with no known exposure to exogenous hormones. At 9.5 years of age the right subareolar mass was excised; histologic examination revealed fibrous breast tissue. Subsequently pubic hair appeared. At 10.7 years of age, the patient complained of right inguinal pain after a minor injury. Examination revealed a tall (height age 12.7 years), mature, muscular boy with enlarged (R: 5 x 3 x 2 cm; L: 3 x 2 x 3 cm) firm, irregular testes, Tanner stage II pubic hair, and modest axillary hair. No perioral pigmentation was present. Testicular ultrasonography revealed multilobular echogenic foci with calcifications. Bone age was 13 years, the LH and FSH secretory responses to GnRH were minimal (LH: < 0.038-->0.28 mIU/ml; FSH: < 0.063-->0.11 mIU/ml), and basal serum testosterone (< 10 ng/dl) and estradiol (< 10 pg/ml) values were undetectable. Following administration of human chorionic gonadotropin (hCG), the serum testosterone concentration increased to 275 ng/dl, while estradiol remained unmeasurable. Spermatic vein concentrations of testosterone were undetectable in the basal state and increased after hCG administration. After bilateral orchiectomy, pathologic examination revealed multifocal tumors composed of brightly eosinophilic, large polygonal cells arranged in nests, cords, and clusters within dense connective tissue or mucinous stroma with lamellar calcifications of varying sizes. These pathologic findings were compatible with a large cell calcifying Sertoli cell (sex-cord)tumor of the testes. Testosterone, estradiol, immunoreactive and bioactive aromatase activity were not detectable in the tumor. Thus, both heterosexual (gynecomastia) and isosexual (increased musculature, pubic and axillary hair) precocious puberty may occur in boys with testicular sex-cord tumors.