The term mantle cell lymphoma (MCL) has been introduced recently, to describe lymphomas related to the primary B follicle and the mantle of the secondary lymphoid follicle. MCL forms a subset of diffuse small cleaved cell lymphomas according to the Working Formulation and correspond closely to the centrocytic lymphomas described in the Kiel classification. They form a distinct clinico-pathological entity and are associated with a specific cytogenetic abnormality; the t(11;14)(q13;q32), which can be detected at a molecular level by rearrangement of the bcl-1 oncogene. As diagnostic criteria for MCL have now been precisely outlined and the clinical presentation described, it should be possible in the future to develop rational treatment protocols for this group of patients, who have a notoriously poor prognosis.