Mantle cell lymphoma (MCL) is most frequently diagnosed by the routine histological (HP) and immunohistochemical (IH) examination. Herein we present a case of 47-yr-old female with general lymphadenopathy and leukemic blood picture. She was initially diagnosed with marginal zone lymphoma (MZL). This diagnosis was based not only on the HP/IH examination but also on the presence of IgM paraproteinemia. Flow cytometry (FCM) examination was helpful in making of the final diagnosis of MCL. Fluorescence in situ hybridization and cyclin D1 mRNA detection by RT-PCR additionally confirmed the MCL diagnosis. The IgM paraproteinemia is rare in MCL, although is a common feature of lymphoplasmacytoid lymphomas (LPL) and is considered a major characteristic of Waldenström's macroglobulinemia (WM).