We describe a case of low-grade B-cell neoplasm with features overlapping between B-chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL). The patient presented with a 10-year history of stable CLL without any treatment. The peripheral-blood picture was consistent with atypical mixed CLL (French-American-British criteria), whereas the lymph-node histology was more consistent with MCL. Neoplastic cells were strongly positive for surface immunoglobulin M, kappa, CD5, CD20, CD23, and cyclin D1. Expression of CD11c was weak. Translocation (11;14) and der(10)t(10;?)(p11;?) were the primary cytogenetic changes observed in both peripheral blood (47%) and lymph node (7%). Trisomy 12 was absent. Deletion 6q21 and rearrangements involving 1p/q, consistently associated with progression in lymphomas, also were noted in the peripheral blood but were nonclonal. The present case and similar cases with features overlapping between CLL and MCL most likely represent hybrids. In cases with features of typical CLL, t(11;14) is probably associated with gradual progression and may precede clinical and histologic transformation.