Six patients had blood and bone marrow manifestations characterized by the presence of morphologically immature or blastic B-lineage lymphoid cells expressing CD5 antigen. The median patient age was 70 years, and the male-to-female ratio was 5:1. The presence or degree of lymphadenopathy and splenomegaly was variable among this group at staging evaluation, although two patients did not have these features. One patient had an antecedent diagnosis of classical nodal mantle cell lymphoma, without prior morphologic blood or bone marrow involvement. Other patients lacked a history of underlying lymphoproliferative disorders. The median white blood cell count was 120 ؋ 10 9 /L. Most patients had thrombocytopenia, whereas only one patient had neutropenia at presentation. Leukemic peripheral blood cells in these six cases were small to medium in size with fine or granular nuclear chromatin and small or inconspicuous nucleoli. The pattern of marrow involvement was interstitial or diffuse, with cells showing immature nuclear features resembling acute leukemia or blastic lymphoma. All tumors demonstrated a consistent immunophenotype of B-cell lineage, surface immunoglobulin positivity, and CD5 antigen expression. The progenitor cell-associated markers CD34 and TdT were not expressed, and CD23 antigen was either negative (three of four cases) or only weakly present (one of four cases). The presence of a karyotypic t(11;14)(q13;q32) was documented in one tumor, whereas two other cases had BCL-1 gene rearrangements by either polymerase chain reaction or Southern blot analysis. Cyclin D1 mRNA overexpression was noted in three of four cases tested. This patient group was characterized by very poor overall survival (median, 3 months; range, 0.5 to 6 months). The aggregate clinical, pathologic, and genetic data in these unusual cases are consistent with de novo or predominant leukemic presentations of blastic mantle cell lymphoma. Accurate diagnosis in such cases is greatly facilitated by cytogenetic studies or the demonstration of BCL-1/ cyclin D1 abnormalities. The expression of surface immunoglobulin (SIg) generally signifies a mature B-cell phenotype and is usually accompanied by nuclear changes of chromatin condensation. However, exceptional lymphoid neoplasms characterized by blastic cytology and mature phenotype have been described. One example is a previous report of nine cases of "mature" B-cell acute lymphoblastic leukemia (ALL), characterized by non-L3 leukemic blast morphology and the presence of SIg positivity (1). These cases were phenotypically somewhat heterogeneous, although TdT was consistently negative in each case and CD10 expression was observed in all of five tumors tested for this antigen. Among these unusual blastic, SIg-positive leukemic cases, karyotypic analysis also revealed heterogeneity. Two tumors exhibited 8(q24) region translocations, implying C-MYC gene deregulation. Of interest, one of these lesions had a t(14;18) anomaly, in addition to 8(q24) rearrangement, suggesting blastic transformation of...