A case of conventional congenital mesoblastic nephroma is presented. The characteristic cytologic findings were scant cellularity, a clean background and a predominant cell population consisting of naked spindle cells arranged singly or in tight, cohesive fragments. Also present were isolated epithelial structures--e.g., tubules and glomeruli. They represented native renal structures that were entrapped by the tumor as it grew. These tubular and glomerular structures may be mistaken for the epithelial component of Wilms' tumor. The cytologic differences between congenital mesoblastic nephroma and Wilms' tumor are discussed.