Purpose: A number of studies have indicated that the tumor proliferation marker MIB-1 and cell cycle inhibitor p27Kip1 expression are of prognostic importance in a variety of cancers. The present study was performed to evaluate the prognostic value of these molecules in Wilms' tumors. Experimental Design: MIB-1 and p27Kip1 expressions were investigated by the means of immunohistochemical analysis of 62 Wilms' tumor. Patients were preoperatively treated by chemotherapeutic agents and had a mean follow-up of 5.7 years. Results: MIB-1 and p27Kip1 were expressed in normal kidney tissues and in the three main components of Wilms' tumor, i.e., the blastemal, epithelial, and stromal cells. In Wilms' tumors, the percentage of MIB-1-positive cells in the blastema ranged between 0 and 42% (mean, 9.4%) and in the epithelial component between 0 and 53% (mean, 19.9%), with a significant difference (P < 0.01). The percentage of blastemal p27 Kip1-positive cells ranged between 3 and 85% (mean, 55.1%) and for the epithelial component between 1 and 87% (mean, 59%). There was a significant inverse relationship between blastemal MIB-1 and p27Kip1 expression in Wilms' tumor. Univariate analysis showed that blastemal MIB-1 and p27 Kip1 expression were indicative for clinical progression and tumor-specific survival. In a multivariate analysis, blastemal MIB-1 and p27Kip1 protein expression proved to be an independent prognostic for clinical progression besides stage. Conclusions: It was concluded that both MIB-1-based proliferative activity and p27Kip1 protein expression in the blastema have prognostic impact in Wilms' tumor.