The immunologic phenotypes of 59 cases of diffuse, aggressive, non-Hodgkin's lymphomas were determined using a battery of immunologic and cytochemical techniques. Included were cases of diffuse, large cell "histiocytic," mixed cell, undifferentiated non-Burkitt's. Burkitt's lymphoma, lymphoblastic lymphoma, and mycosis fungoides/Sézary's syndrome were excluded from this study since these are distinct clinicopathologic entities with well-recognized immunologic phenotypes. The immunotype could be determined in 57/59 (97%) cases tested: 31 of 59 cases (53%) were B-cell type, 25 of 59 (42%) were peripheral T-cell type, and one was true histiocytic. Two cases had no detectable markers and were called "null cell." This relatively high frequency of peripheral T-cell lymphomas in an American series previously has not been observed and may be a result of progressive improvements in immunologic techniques. Monoclonal anti-T cell antibody staining was performed in 11 T-cell cases and corroborated the findings using spontaneous E-rosette formation. Eight of the T-cell lymphomas had a helper cell phenotype whereas one had a suppressor cell phenotype and two could not be subclassified. All B-cell lymphomas in this series possessed monoclonal surface immunoglobulin detected by direct immunofluorescence of viable cells. Enzyme cytochemistry profiles only partially correlated with immunotype and were not believed to be helpful in the determination of specific phenotypes. There were no significant differences between the B-cell and T-cell diffuse aggressive lymphomas with respect to sex, constitutional symptoms, stages, sites of extranodal involvement, complete remission rate, or survival when they were studied prior to the initiation of aggressive therapy. Although immunotyping can be successfully performed in essentially all cases of diffuse, aggressive non-Hodgkin's lymphomas, to date, the authors have been unable to demonstrate that immunotype alone has an independent prognostic effect.