T-cell non-Hodgkin's lymphomas can be considered the neoplastic equivalents of immunologically functional, site-restricted T lymphocytes. Little is known about the occurrence and clinical behavior of T-cell lymphomas that are the neoplastic equivalents of different functional T-cell subsets. Here, we investigated the prevalence, preferential site, immunophenotype, and clinical behavior of the neoplastic equivalents of activated cytotoxic T cells (CTLs) in a group of 140 nodal and extranodal T-cell lymphomas. Activated CTLs were shown immunohistochemically with a monoclonal antibody against granzyme B, a major constituent of the cytotoxic granules of activated T cells. Granzyme B-positive T-cell lymphomas were mainly found in mucosa- associated lymphoid tissue (MALT; nose, 63% of the cases; gastrointestinal tract, 46%; and lung, 33%). Granzyme B-positive cases with primary localization in MALT were more often associated with angioinvasion (P = .005), necrosis (P = .002), and histologic characteristics of celiac disease in adjacent mucosa not involved with lymphoma. Eosinophilia was more often observed in granzyme B-negative cases (P = .03). Most cases belonged to the pleomorphic medium- and large-cell group of the Kiel classification. CD30 expression was more often found in granzyme B-positive lymphomas of MALT (P = .04), whereas CD56 expression was exclusively found in nasal granzyme B-positive lymphomas. Immunophenotypically, most of the cases should be considered as neoplastic equivalents of activated CTLs based on the presence of T- cell markers on tumor cells. In two cases of nasal lymphoma, tumor cells probably were the neoplastic counterparts of natural killer cells. The prognosis of the granzyme B-positive gastrointestinal T-cell lymphomas was poor but did not differ from granzyme B-negative gastrointestinal T-cell lymphomas. This indicates that, in peripheral T- cell lymphomas, site of origin is more important as a prognostic parameter than derivation of activated CTLs.