Predominantly cystic renal neoplasms have been the source of diagnostic confusion and controversy. In this review, the authors analyze the clinical and pathological features of four entities that consistently exhibit a diffusely cystic growth pattern, are strikingly similar in their gross appearances, and are not separable by preoperative imaging studies. Based on the literature, this review concludes that tumors in young children that have been classified as cystic nephroma and cystic partially differentiated nephroblastoma likely represent a single entity, and all should be considered highly cystic Wilms' tumors with little or no capacity for invasion or metastasis and diagnosed as cystic partially differentiated nephroblastoma. Conversely, cystic nephroma in adults has no discernible connection with Wilms' tumor or nephrogenic rests and should be considered a benign composite neoplasm of stroma and epithelium of unknown histogenesis, which may rarely become malignant with secondary development of a sarcoma. Multilocular cystic renal cell carcinoma appears to be unrelated to cystic nephroma and if the following criteria are met, it appears to be a neoplasm with an intrinsically cystic growth pattern, and no, or at most little, malignant potential: (1) an expansile mass is surrounded by a fibrous wall, (2) the interior of the tumor entirely is composed of cysts and septa with no expansile solid nodules, and (3) the septa contain aggregates of epithelial cells with clear cytoplasm. Cystic hamartoma of the renal pelvis is a rare, complex tumor composed of stroma with a prominent smooth muscle component and a variety of epithelial elements.