We studied the morphologic, immunologic and clinical features of 14 cases of primary non-lymphoblastic non-Hodgkin's lymphomas of the mediastinum. The patients ranged in age from 3 to 76 years, with a median age of 28 years. According to the Ann Arbor classification, 71% of our cases were in an early stage. Three cases were in Stage I, eight in Stage II, one in Stage III and two in Stage IV (one with multiple hepatic lesions and another with bone marrow involvement). The patients were heterogeneous in terms of the disease and were therefore histologically classified into three categories: diffuse large B cell lymphoma with sclerosis (DLS; n = 8); large cell anaplastic lymphoma (LC-Ana; n = 5); and low grade B cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma; n = 1). DLS was the most common group and was characterized as CD5-, CD10-, CD19+, CD20+, CD21- and CD22+. Imprint smears showed azurophilic granules in the cytoplasm of the tumor cells of three of four DLS cases. All of the six cases examined were negative when tested for Epstein-Barr virus (EBV) sequences after hybridization with the EBV internal repeat probe. DLS and MALT lymphoma cases were of a B-lineage lymphoma of the thymus, while most of the LC-Ana cases were of a T-lineage lymphoma. Patients with non-lymphoblastic non-Hodgkin's lymphomas had a relatively favorable prognosis compared with lymphoblastic lymphoma (P < 0.01 by the generalized Wilcoxon test). There was no significant difference in the survival between non-lymphoblastic non-Hodgkin's lymphoma and Hodgkin's disease (P > 0.05 by the generalized Wilcoxon test).