Immunologic phenotyping of lymphoblastic lymphomas has shown that most of these are tumors of T-cell origin. In this report, we describe two patients with biopsy-proven lymphoblastic lymphoma whose tumors had no T-cell markers when tested by immunoperoxidase with a large panel of monoclonal antibodies. However, the tumor cells did express the common ALL antigen (CALLA), la antigen, and a 24,000 dalton ALLassociated antigen defined by monoclonal antibody DU-ALL-1. The tumor cells also lacked surface immunoglobulin. Although this phenotype is that seen in most cases of acute lymphoblastic leukemia, the patients were never leukemic at any time during their clinical course. Our results support the overall similarity between lymphoblastic lymphoma and acute lymphoblastic leukemia. Further, they suggest that it may be possible to identify prognostically significant immunologic subtypes of lymphoblastic lymphoma.