Chronic lymphoproliferative disorders sometimes can be difficult to classify. We report 4 cases characterized by large cells with distinct central nucleoli, reminiscent of prolymphocytic leukemia, but shown on further workup to represent mantle cell lymphoma. At initial examination, the patients had generalized lymphadenopathy, splenomegaly, and a leukemic blood picture. The peripheral blood showed many large cells with round to slightly irregular nuclei, single central nucleoli, and a fair amount of pale cytoplasm. The picture was not typical of prolymphocytic leukemia because of the presence of generalized lymphadenopathy and the large size of the circulating abnormal cells. Immunophenotypic study showed that the large lymphoid cells were CD5+ CD23- mature B cells with overexpression of cyclin D1, and cytogenetic study demonstrated the translocation t(11;14)(q13;q32) in 3 patients. Lymph node biopsy confirmed a diagnosis of mantle cell lymphoma, pleomorphic variant, in all 4 patients. This study documents the existence of an unusual leukemic form of mantle cell lymphoma with prominent nucleoli; the clinicopathologic features that distinguish it from other chronic lymphoproliferative disorders are discussed.