Multilocular cystic renal cell carcinoma (MCRCC) appears to be a distinct subtype of renal cell carcinoma with characteristic gross and microscopic features. The authors' ten-year experience (1977-1987) included six cases of MCRCC that were followed for a minimum of two years, with neither recurrence nor metastasis observed in any of the cases. During this period, there were 855 urologic procedures for the upper urinary tract, with 256 neoplasms or cysts identified. These included 32 simple cysts, 41 transitional cell carcinomas, 133 renal cell carcinomas, 17 papillary renal cell carcinomas, and 33 miscellaneous tumors. Histologically, the MCRCCs were well-demarcated multicystic lesions containing variably sized aggregates of neoplastic clear cells showing grade 1 nuclear features and little or no mitotic activity. The cyst walls were densely fibrotic, and the lining was often devoid of epithelium. Flow-cytometric analysis performed in five of the six cases with the use of paraffin-embedded tissue showed the tumors to be diploid in all instances, with low proliferative activity. The authors believe that this tumor is a low-grade variant of renal cell carcinoma and should be studied further to determine appropriate therapy.