A detailed ultrastructural description of the cellular variant of congenital mesoblastic nephroma (CMN) is presented and compared to the classical form. Studied were 9 cases of the cellular variant, 6 mixed (cellular/classical) tumors, and 1 classical CMN. The occurrence of a broad selection of ultrastructural features was assessed using a semiquantitative scoring system. The results indicate that cellular CMNs are composed mainly of primitive mesenchymal cells, but also usually contain varying numbers of differentiating fibroblasts and myofibroblasts. This entity thus bears a closer resemblance at the ultrastructural level of organization to infantile fibrosarcoma than to conventional fibrosarcoma. Electron microscopy can be useful in distinguishing this relatively benign entity from the several malignancies with which it is sometimes confused.