Local immunocompetence in patients with bladder tumors was evaluated immunohistochemically using monoclonal antibodies. As well as investigating the subsets of peripheral blood lymphocytes by flow cytometry, the subpopulations of tissue-infiltrating lymphocytes were investigated in frozen specimens from bladder cancers (n = 25) and from noncancerous tissues (n = 4). Six serial sections were stained with Leu4, Leu7, Leu 10, LeuM3, OKT4, and OKT8 antibodies, respectively. Marked lymphocytic infiltration into bladder tumors was a favorable prognostic sign. T cell infiltration was marked in the noninvasive group and was prominent around tumor cells, whereas B cells were rare in the stroma. T cells predominated over B cells in 19 of 25 bladder tumors. In patients with low-stage tumors, OKT8 cells were more prominent than OKT4 cells. NK cells accumulated around the cancer cells and infiltrated within cancer nests; NK cell infiltration was scanty in high-stage cases. There was a slight scattering of macrophages in 13 cases. In contrast to the findings in subsets of tissue-infiltrating lymphocytes, subsets of peripheral blood lymphocytes did not correlate with stage of disease. These findings suggest the presence of local immunosurveillance against bladder tumors. To elucidate host immune response against tumor, it may be important to characterize tissue-infiltrating lymphocytes.