Tumor progression and clinical outcome for patients with renal cell carcinomas (RCCs) cannot be predicted based solely on tumor staging and grading. In a retrospective study we have therefore attempted to analyze the capacity of proliferation markers to provide additional prognostic information. One hundred seven cases of RCC were investigated by immunohistochemical analysis using two different monoclonal antibodies: PC10, which recognizes a proliferating cell nuclear antigen (PCNA), and MIB-1, which identifies the Ki-67 antigen in formalin-fixed, paraffin-embedded material. PCNA frequency ranged from 0% to 71% (mean, 17%), and MIB-1 expression, from 0% to 43% (mean, 11%). PCNA scores correlated significantly with MIB-1 immunoreactivity. PCNA and MIB-1 immunoreactivity showed a significant correlation with tumor grade. A strong correlation was also observed for T-component of stage and MIB-1 scores, but no correlation was found between PCNA and T-component of stage. In univariate analysis, PCNA immunoreactivity and MIB-1 scores were significant predictors of survival. Multivariate analysis, using a Cox proportional hazard model, showed PCNA index, N-component of stage, and tumor grade to be independent predictors of tumor progression, which is not the case for MIB-1 index.