Peripheral T-cell lymphoma (PTCL) forms a morphologically heterogeneous group of non-Hodgkin's lymphomas (NHL) with distinct immunophenotypes of mature T cells. Progress has been slow in defining specific clinicopathological entities to this particular group of NHL. In order to elucidate the specific characteristics of PTCL, a direct comparison of PTCL with a group of diffuse B-cell lymphomas (DBCL) was performed. Between June 1983 and December 1987, we studied 114 adults with NHL, using a battery of immunophenotyping markers. Adult T-cell leukemia/lymphoma, lymphoblastic lymphoma, mycosis fungoides/Sézary syndrome, follicular lymphoma, well-differentiated lymphocytic lymphoma, and true histiocytic lymphoma were excluded from this study since these are distinct clinicopathologic entities with well-recognized immunophenotypes. Of the remaining 75 patients, 70 who had adequate clinical information were analyzed, and of these, 34 were PTCL and 36 were DBCL. Classified according to the National Cancer Institute (NCI) Working Formulation (WF), 68% of PTCL and 31% of DBCL were high-grade lymphomas. Clinical and laboratory features were similar, except PTCL had a characteristic skin involvement and tended to present in more advanced stages with more constitutional symptoms. Induction chemotherapy was homogeneous in both groups, and complete remission rates were 62% for PTCL and 67% for DBCL. Patients with DBCL had a better overall survival than patients with PTCL, but the survival benefit disappeared after patients were stratified according to intermediate- or high-grade lymphoma. A subgroup of PTCL patients who had received less intensive induction chemotherapy was found to have a very unfavorable outcome. We conclude that (1) PTCL follows the general grading concept proposed in WF classification; (2) within a given intermediate or high grade, PTCL and DBCL respond comparably to treatment; (3) the intensity of induction chemotherapy has a crucial impact on the outcome of PTCL patients; and (4) with a few exceptions, the clinical and laboratory features of PTCL and DBCL are comparable.