[A case report of renal cell carcinoma in a renal cyst].

We report a case of renal cell carcinoma within a simple renal cyst in the upper pole of the right kidney. The renal cyst was found incidentally by ultrasonography. During the 3 years of follow-up, the cyst size was increasing and a small solid mass was arising from the cyst wall. Angiography revealed a hypervascular tumor stain in the renal cyst. At operation a 15 mm tumor arose from the wall of the cyst. Histopathological examination showed clear cell type renal cell carcinoma and "cyst" with sheets of carcinoma within the cyst wall. The coexistence of renal cyst and tumor is considerably rare. Fifty two cases were collected from the Japanese literature including this, and they are reviewed briefly.