Recent data from studies of experimental murine tumors and certain human tumors (primarily melanoma) suggest that tumor-infiltrating T-lymphocytes (T-cell TILs) represent a highly potent and specific host antitumor response. We conducted an immunohistochemical analysis of the T-cell TIL subpopulations in frozen tissue sections taken from 82 consecutive B-cell diffuse large cell lymphoma (DLCL) patients. Initially, we analyzed the relationship in these patients between relapse-free survival (RFS) and their T-cell TIL characteristics. Nineteen patients had a low percentage (less than 6% of Leu-2+ (suppressor/cytotoxic) T-cell TILs, and 63 patients had a high percentage (greater than 6%) of Leu-2+ TILs. We found that a low percentage of Leu-2+ TILs correlated with a reduction in RFS: at 20 mo follow-up, all 19 low Leu-2+ patients had relapsed, whereas 70% of the 63 high Leu-2+ patients remained relapse-free (P = 0.008). No significant correlations appeared between patients' T-cell TIL subsets and overall survival. The percentage of newly diagnosed tumors with low counts of Leu-4+ (pan-T) TILs was marginally greater among interleukin-2 (IL-2) receptor-positive tumors than among IL-2 receptor-negative tumors (50 versus 28%, P = 0.098), which suggests that specific phenotypic characteristics of B-cell DLCL may modulate the host T-cell TIL response. Our results indicate that the host's T-cell TIL response in B-cell DLCL can be quantitated from frozen tissue sections and that this response may be related to disease course. Further related TIL studies may lead to new immunorestorative therapeutic approaches for patients with deficient or aberrant cytotoxic T-lymphocyte host responses.