A group of nineteen primary renal tumors in childhood was analyzed by flow-cytometry. All clinically important entities were included. Paraffin embedded material was used. Two anaplastic nephroblastomas and a case of renal rhabdomyosarcoma had an aneuploid DNA content. One Wilm's tumor showed a moderate DNA-hyperdiploidy. One of two rhabdoid tumors showed a marked right-sided shift of G1/G0 phase from that of a control and was considered as probably hyperdiploid. All other tumors including high grade neoplasms such as rhabdoid tumor and bone metastasizing tumors were found in the DNA-diploid range. Proliferation index was high in most tumors and in six it reached or exceeded 40. Mitotic index counted in the histological sections correlated with the percentage of G2 + M phase in most cases. Although small, the group of tumors under study shows that prognostic criteria cannot be based on the DNA content or on the proliferation index only. The only relevant result may be an overt DNA-aneuploidy in anaplastic nephroblastomas. Difficulties in interpreting the results of flow cytometry are discussed.