Lymphocyte subsets were examined in renal cell carcinoma (TILs), adjacent non-tumor renal tissue and peripheral blood (PBLs) by flow cytometry and histochemistry in eighteen patients with renal cell carcinoma. CD5-positive cells were predominant in the TILs in 14 patients. In the renal cell carcinoma tissue, CD8-positive cells were predominant over CD4-positive cells, resulting in a less than unity ratio of CD4/CF8-positive cells. The lymphocyte number was significantly in adjacent normal renal tissue than in renal cell carcinoma. However, lymphocyte subsets ratios were not significantly different between these two tissues. PBLs showed the same proportions (CD4/CD8 mean 1.9 +/- 0.8) as the previously published healthy controlled data. The proportions of CD8-positive cells were significantly increased (p less than 0.05) and those of CD4-positive cells were also significantly decreased (p less than 0.01) in the TILs. The infiltrating pattern of TILs in 17 patients was divided histochemically into cluster (N = 7), single (N = 4), and mixed types (N = 6). The cluster and mixed types were significantly more common in grade 1 tumors and the single type was more common in the grade 2 tumors (p less than 0.05). The pT3 tumors showed the single type of TIL infiltration pattern, but showed no significant difference. In the cluster pattern of TILs, CD8-positive cells were surrounded by CD4-positive cells. Non-tumorous kidneys showed no infiltration of lymphocytes, except in 2 patients of pyelonephritis. These results suggest that cytotoxic T-cells stained as CD8 play an immunoreactive role against renal cell carcinoma.