Benign mixed epithelial and stromal tumor of the kidney has only recently been recognized as a distinct clinical and pathological entity.1 Approximately 38 cases have been iden- tified in the literature. We report an unusual case of mixed epithelial and stromal tumor that may be of mullerian deri- vation. A 51-year-old woman with an incidental renal mass de- tected on ultrasound underwent computerized tomography which revealed an enhancing, inhomogeneous 16 12 10 cm. mass in the left kidney (fig. 1). The mass contained irregular small cystic pockets and peripheral punctate calci- fications. There was an ill-defined mobile mass in the left upper quadrant on physical examination, and the patient underwent left radical nephrectomy for presumed cystic re- nal cell carcinoma. Gross pathological examination revealed a 13 8 7 cm. white multilocular cystic tumor filled with mucoid fluid. The cysts were small, less than 1 cm., with thin walls and no significant solid component. The tumor replaced the upper pole and projected into the renal pelvis but was confined within the renal capsule. Microscopically the tumor was com- posed of a mixture of epithelial and stromal elements. Cystic dilated spaces were lined by an admixture of mucus produc- ing columnar and ciliated serous epithelium (fig. 2). Papillary projections containing fibrovascular cores were seen extend- ing into these cystic spaces. There was no cytologic atypia or mitotic activity observed in the epithelial lining. The epithe- lial elements were strongly decorated by antikeratin stain CAM 5.2 and carcinoembryonic antigen stain was negative. The immunophenotype of the subepithelial stromal elements was vimentin positive, desmin negative, muscle specific actin (HHF35) negative, smooth muscle actin negative, CD34 neg- ative, S100 negative, estrogen receptor protein focal positive and progesterone receptor protein focal positive. The glandu- lar epithelium also displayed strong positivity for estrogen