Mantle cell lymphoma (MCL) is a B-cell neoplasm characterized by aggressive clinical course with an average 3- to 5-year patient survival. We present a patient whose illness turned from initial classical morphological variant to a more aggressive pleomorphic form of MCL in only a few months, but with unchanged long-term indolent clinical course. At the time when lymphoid cell pleomorphism was proven, the disease presented itself through recurrent peripheral lymphadenopathy without extranodal involvement or general symptoms. Other numerous abnormalities were found next to typical cytogenetic translocation t (11,14). Histopathology confirmed the diagnosis of MCL, pleomorphic type. After autologous stem cell transplantation, the disease remained morphologically the same, but the patient was in a good general condition for a long period of time. More than two years after the pleomorphic MCL had been diagnosed and one year after the transplantation, major lymphadenopathy occurred. Our case report points to a large spectrum of morphological and cytogenetic variability of MCL, which often does not correlate with the clinical course of the disease.