Clinicopathologic features of 17 cases of "cystic partially differentiated nephroblastoma" (CPDN) including three seen in personal experience have been reviewed. CPDN has previously been designated by various terms such as "benign multilocular cystic nephroma," "polycystic nephroblastoma," and "cystic nephroma." The term CPDN is preferred because it aptly denotes the characteristic gross and microscopic features and the probable histogenesis of the lesion. CPDN occurs before 2 years of age. Clinically, abdominal mass is the most common presenting feature. CPDN is often diagnosed as Wilms' tumor on clinical and radiologic grounds. Grossly, the lesion is almost totally or predominantly multicystic. Microscopically, the cysts are usually lined by flattened to cuboidal epithelium. The septa contain a mixture of blastemal cells and poorly differentiated and well-differentiated epithelial and mesenchymal elements. It is this feature which distinguishes CPDN from multilocular cyst of the kidney. Although there is a suggestion that there might be two subtypes of CPDN-one with predominantly well-differentiated elements and the other with predominantly poorly differentiated elements-no definite conclusions can be drawn regarding the existence of such subtypes on the basis of available data. Arguments could be advanced in favor of both the neoplastic and dysgenetic nature of CPDN. However, the view that CPDN is a neoplastic lesion is favored. On the basis of available data, CPDN appears to take a benign course, and simple nephrectomy seems to be the treatment of choice. However, in view of the possibility of recurrence as shown by rare cases of congenital mesoblastic nephroma (another usually benign lesion in the spectrum of infantile renal necoplasia), regular follow-up is recommended.