Two patients with Simple Renal Cysts were studied and followed along a three years period. The first case was that of a 3 months old infant with a right abdominal mass, that had been discovered in the first month of life, and severe, persistent arterial hypertension that did not respond to treatment with propanolol, diazoxyde and hydrochlorothiazide: A solitary cyst without communications to the renal excretory apparatus was confirmed at surgery. The second patient had a left hemiabdominal mass that was detected at the 5th month of gestation, during a rutine echographic examination of his mother. In both cases a neprectomy was performed. Arterial hypertension subsided to normal two months after operation in case 1 and both patients were healthy at 42 months of age. (