MCLs are thought to arise from a subset of B cells that normally express the CD5 antigen and that reside in the mantle zone of secondary lymphoid follicles. Although expression of the CD5 antigen is also seen in small lymphocytic lymphoma and chronic lymphocytic leukemia, MCL differs from SLL/ CLL in several ways. Whereas trisomy of chromosome 12 is the hallmark cytogenetic abnormality of SLL/CLL, the translocation (11;14) (q13q32) is the most frequent karyotypic abnormality in MCL. The histologic pattern of MCL is most frequently diffuse. However, this lymphoma can grow in a unique pattern called 'mantle zone MCL,' indicating that the malignant cells expand the mantle of the follicle and grow around a normal germinal center. If the germinal center is also replaced by the malignant cells, but the follicular architecture remains, the pattern appears nodular. The clinical presentation of MCL is usually only seen with advanced disease stage, particularly in patients with diffuse MCL. The bone marrow is the most frequently affected extranodal site, followed by the gastrointestinal tract. The histologic pattern of disease in the lymph nodes correlates with clinical outcome. Patients with diffuse MCL have poor response to frontline combination chemotherapy including doxorubicin, whereas patients with mantle zone MCL have excellent complete remission rates. The therapeutic response correlates in turn with worse survival outcome for patients with diffuse MCL compared to mantle zone MCL. The few patients with nodular MCL had clinical behavior similar to diffuse MCL. The chemotherapeutic response of diffuse and nodular MCL, however, is quite poor, and we would propose that new investigational approaches be considered in the front-line therapy of these disorders.