The relative abundance of tumor-infiltrating mononuclear cells (TIM) was retrospectively evaluated by immunohistology in nephrectomy specimens from 24 cases of human renal cell carcinoma (RCC). The extent of T-lymphocyte (T-cell) and B-lymphocyte (B-cell) infiltration of the tumors was graded semiquantitatively (0 to 4) in each case. Results of this analysis were correlated with clinical evidence of recurrent RCC, histologic evidence of venous invasion, tumor necrosis, and histologic cell type. Clinical follow-up demonstrated the presence of recurrent tumors in four of the 24 cases (17%), and these cases correlated with higher stage and pathologic grade tumors, as well as significantly increased TIM. Venous invasional and tumor necrosis correlated directly with tumor size, pathologic grade, and extent of TIM. Venous invasion was also associated with advanced stage. The group of cases with mixed or granular histologic cell type was associated with advanced stage, high pathologic grade, and increased T-cell infiltration compared to the clear-cell group of tumors. Overall, the pathologic grade demonstrated the highest correlation coefficients with clinical and TNM stage, but also correlated directly with the extent of T-cell infiltration of the tumor. These results confirm previous findings demonstrating a correlation in RCC between increased T-cell infiltration and both high clinical stage and pathologic grade. In addition, increased T-cell infiltration was found to correlate with tumor recurrence, indicating that such infiltration, along with high pathologic grade, is another indicator of poor prognosis in RCC.