We report a case of renal cell carcinoma within a renal cyst. A 45-year-old man visited our hospital with the chief complaint of left back pain. Ultrasonography was examined by a physician, and it revealed a right renal cyst by chance. Computed tomography demonstrated a thick wall in the cyst which showed enhancement of density with contrast medium. The physician followed the cystic lesion periodically, by ultrasonography and computed tomography. The cyst on volume did not change, but the thick wall had a tendency to develop to inner space. The patient was referred to our department. Angiography was performed, and it showed neovascularity and tumor staining. We diagnosed the renal tumor in a renal cyst. Transabdominal right radical nephrectomy was performed on 18 June, 1991. The resected specimen involved a mall tumor (2.5 x 2.0 cm) in the cyst. Histological examination of the tumor was renal cell carcinoma which was well capsulated by connective tissue without invasion to normal parenchyma and perirenal fat. We speculated that the renal cell carcinoma originated from the cyst wall and it developed to inner cystic space. The natural history of the renal cell carcinoma in a renal cyst is not clear. Our case shows a type of its history. We reviewed and discussed the types and diagnosis of the renal cell carcinoma in a renal cyst.