In this report we describe the results of a clinical and immunohistochemical analysis of 11 consecutive patients with the specific clinicopathologic entity of Lennert's lymphoma (non-Hodgkin's malignant lymphoma with a multifocal epithelioid histiocytic reaction IMLEH]) evaluated at the Arizona Cancer Center. Detailed immunophenotyping of ten patients showed that seven patients (73%) had an activated "novel" T-cell phenotype, indicative of peripheral T cell lymphoma (PTL). Additionally, six of these seven PTL patients had T-helper (Leu-3) antigen expression to the exclusion of T-suppressor (Leu-2) expression. Three patients, in complete contrast, had a B-cell lymphoma with monoclonal immunoglobulin expression. The B-cell MLEH were morphologically indistinguishable from T-cell MLEH. Clinically, the initial diagnosis proved difficult; ten of the 11 patients were initially misdiagnosed, most often as another lymphoid disorder or as granulomatous disease (mean delay of 10 months in diagnosis from onset of symptoms). The median survival of all patients was 20 months (1 to 45+ months) with two apparent subgroups: those who had rapid progression of disease with a median survival of 5 months, all of T-cell phenotype; and a small group whose median survival has not yet been reached, all of B-cell phenotype. Our results suggest that the immunophenotype, B-cell versus T-cell, may be a major predictor of survival, with B-cell MLEH patients having a longer survival than those of T-cell type.