Diffuse aggressive non-Hodgkin's lymphomas are immunologically and morphologically heterogeneous. Morphologic subclassifications have been proposed with a presumptive relationship to immunologic phenotype. Six histologic types were studied, four of presumed follicular origin (large cleaved, large non-cleaved, mixed follicular center cell, and blastic) and two of presumed nonfollicular origin (pleomorphic pyroninophilic and pleomorphic convoluted). Two of these follicular subtypes were found to be associated with the frequent presence of B-cell markers; large non-cleaved, six of seven; and blastic, four of six. However, four of nine T-cell lymphomas were misclassified as lesions of follicular center cell type, indicating the difficulty of distinguishing some convoluted cells of T-cell lymphoma from irregularly cleaved cells of follicular lymphoma. Pleomorphic pyroninophilic or B-immunoblastic morphology was associated with B-cell markers in only two of six cases; two additional cases had T-cell markers, one had histiocytic markers and in one case, no markers were demonstrable. Thus, this morphology may reflect a common pathway of transformed lymphoid cells. Some cases of T-cell origin could be recognized by the striking pleomorphic convoluted histology (three of five cases) but morphologic overlap with pleomorphic B-cell tumors, particularly transformed nodular lymphomas, posed a problem. This histopathologic subclassification correctly predicted immunologic phenotype in only 61% of cases, suggesting that in diffuse non-Hodgkin's lymphomas, histologic appearance alone may not be a reliable indicator of immunologic surface markers.