We describe morphological, immunophenotypic, and cytogenetic characterization of a case of multiple myeloma (MM) that showed plasmablastic transformation at the terminal phase with a picture resembling acute leukemia. The plasmablasts expressed monotypic cytoplasmic immunoglobulin together with myeloid and megakaryocytic markers at disease transformation. Conventional cytogenetic study of bone marrow cells showed coexistence of hypodiploid and hyperdiploid cells, with the former being the predominant clone as evidenced by an interphase fluorescence in situ hybridization study. The clinical course in our case shows that plasmablastic transformation should be considered in the differential diagnoses of disease progression in MM. Whether de novo plasmablastic myeloma and plasmablastic transformation can be distinguished as a progression from underlying MM merits further investigation, especially in terms of biologic features and relevance to prognosis.