Purpose Ki-67, tumor proliferation marker, is an important prognostic factor in a variety of cancers. In the present study, we investigated the expression and the prognostic value of Ki-67 in nephroblastoma. Methods Ki-67 expressions were investigated by immunohistochemistry on paraffin-embedded material in 48 children operated on because of nephroblastoma. Patients were treated according to SIOP protocol. The mean follow-up period was 5.4 years. A proliferation index was obtained by immunohistochemistry using anti-Ki-67 anti-body. Results The mean Ki-67 proliferation index in the blastemal type was 12.3%, and in the epithelial type, 21.4%. In the anaplastic type, Ki-67 proliferation index was: in the blastemal component 20%, in the stromal 21%, and in the epithelial 31%. In the mixed tumor type, Ki-67 proliferation index was assessed as: in the blastemal component 10%, in the epithelial 33% and in the stromal 31.5%. Proliferation index for the epithelium was significantly higher than those found for the blastema ( P  = 0.001). A correlation between Ki-67 and tumor stage found proliferation index significantly higher in stages I and II ( P  = 0.002). Conclusion The results support the conclusion that Ki-67 is a relevant marker for assessing the proliferative activity and tumor cell dynamics of nephroblastoma, but it may not be a good clinical prognostic marker.