A 3-year-old boy with a Wilms' tumor had unusually severe hypertension, polydipsia, polyuria and hypokalemia. Physical-examination on admission was unremarkable except for the presence of a smooth, firm mass in the right abdomen. Computerized tomography showed a tumor occupying the upper two thirds of the right kidney. Plasma renin activity and aldosterone concentration were markedly elevated, 37.7 mg/ml/hour(normal in supine position 0.15-2.33 mg/ml/hour) and 120.1 ng/dL(normal in supine position 1 to 16 ng/dL) , respectively. Hypertension varied from 150/90 mmHg to 240/180 mmHg, and was not effectively controlled by antihypertensive drugs. Right nephrectomy was performed on the sixth hospital day. At laparatomy, there was no evidence of mechenical compression of the renal artery by the tumor. The tumor, about 8 cm in diameter, was confined to the renal capsule without involvement of the renal blood vessels at the hilum. Histopathology was Wilms' tumor of favorable histology. On electron microscopy, tumor cells contained intracytoplasmic electron dense secreting graules, suggesting the possibility of renin secreting tumor cells. Shortly after nephrectomy, signs and symptoms were relieved dramatically, and plasma renin activity and aldosterone concentration were also decreased to normal.