We report a case of indolent mantle cell lymphoma with progression to pleomorphic mantle cell lymphoma 8 years after initial presentation. The first lymph node biopsy showed expanded mantle zones composed of uniformly small B lymphocytes. A cyclin D1 immunohistochemical stain was negative and the patient was observed. Eight years later, the patient developed symptomatic splenomegaly. Microscopic examination of the spleen revealed expanded mantle zones with an increased number of large cells with irregular nuclear contours. Immunohistochemistry for cyclin D1 was positive. A repeat cyclin D1 immunohistochemical staining performed on the initial lymph node biopsy was positive, indicating an inadequate initial study. Immunoglobulin heavy-chain gene rearrangement studies confirmed clonal identity. A revised diagnosis of indolent mantle cell lymphoma with progression to pleomorphic mantle cell lymphoma was rendered. The differential diagnosis of mantle cell lymphoma, including clinical and morphologic variants, is discussed.