Isolated primary aldosteronism in a patient with adrenal carcinoma and XY/XXY mosaic Klinefelter's syndrome. 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. 