Although breast cancer, germ cell tumors and other neoplasms are known to occur in patients with Klinefelter's syndrome, adrenal carcinoma has not yet been reported in such patients. We describe a rare case of severe primary aldosteronism as the unique manifestation of a large adrenocortical carcinoma in a patient with Klinefelter's syndrome. Complete biological and hormonal evaluation was performed. Surgical treatment was successful and the patient remained asymptomatic with normal biological and hormonal values after 1 year of followup.