A case of cystic renal mesothelioma in a 55 year-old woman is described. The patient had only intense pain in the left lumbar region. The tumor macroscopically appeared as a cystic multilocular mass, 20 cm. in diameter, sharply demarcated from the renal parenchyma. A light and electron microscope study was performed, and strongly supported mesothelial origin. The patient was treated by surgical therapy which was completely resolutive. The differential diagnosis between this tumor and peri and para-renal cystic lesions is discussed.