Ten renal cell carcinomas in children under 15 years were investigated. The average age was 122.5 months and the girls predominated in our cases (7 girls, 3 boys). By using the classification of Thoenes et al., Pathol Res Pract 181: 125-143, 1986 a predominance of clear cell-eosinophilic tumor cell type and of the tubulopapillary growth pattern was found. Immunohistochemistry revealed a heterogeneity of cytokeratin expression. By using the monoclonal antibodies Cam 5.2 and KL 1, cytokeratins were found in 7 cases each. The other 4 cytokeratin antibodies used were less sensitive. The expression of cytokeratin 13 in 3 cases suggested a more complex histogenesis than assumed. Vimentin was found in 3 tumors, but an association to a higher grade (G) of malignancy was not found in these cases. One tumor expressed the Tamm-Horsfall-protein, which is predominantly found in the distal tubule of the normal kidney. In summary the results of immunohistochemistry characterized the great heterogeneity of these tumors. Follow-up information was available in 9 cases. All patients with G I- and G II-tumors were free of disease after an average time of 39.6 months (mean 27 months). Two of the 3 cases with G III-tumors died after 9 and 15 months, despite additional chemo- or radiotherapy. Therefore tumors of grade I and II of the Thoenes classification seem to have a good prognosis.