A prenatally detected suprarenal cystic mass measuring 2 cm was found to have enlarged upon postnatal ultrasonography at 6 weeks of age. Magnetic resonance imaging showed a 57 x 50 mm mass in the left adrenal region displacing the kidney inferiorly. The infant underwent an adrenalectomy with total resection of the tumor, which proved on histologic examination to be a mature teratoma. Prenatally detected suprarenal masses are likely to be neuroblastoma or adrenal hemorrhage, but may be rare benign lesions such as extralobar pulmonary sequestration, bronchogenic cyst, or renal dysplasia. Although teratoma in the adrenal region is extremely rare, it should be included in the clinical and radiologic differential diagnosis of prenatally detected suprarenal masses. Total excision of the mass for histologic diagnosis is indicated.