Primary nodal marginal zone B-cell lymphoma (MZBCL) is recognized as a rare and distinct entity. The rate of histologic transformation into diffuse large B-cell lymphoma (DLBCL) seems lower than the rate of transformation in follicular lymphoma. We herein report a rare case ofnodal MZBCL showing transformation into DLBCL. The patient was a 73-year-old female withcervical lymphadenopathy. On the initial biopsy, the lymph node architecture was diffuselyeffaced with an extensive interfollicular and parafollicular infiltrate of monocytoid B-cells. Therewere scattered large blastic B-cells without formations of compact sheets. The diagnosis ofnodal MZBCL was made. The patient did not receive chemotherapy and was treated with aconservatively supportive regimen. Forty two months later, the patient developed a new cervicallymphadenopathy and a biopsy was performed. Histologically, the lymph node revealeddiffuse sheets of transformed large B-cells showing prominent nucleoli. The diagnosis ofDLBCL transformed from nodal MZBCL was made. The patient was treated with 3 cycles ofcombined CHOP chemotherapy and she showed clinical improvement. These observationssuggest that an untreated primary nodal MZBCL may undergo high-grade transformation.