jabbrv-ltwa-all.ldf jabbrv-ltwa-en.ldf Background Plasmablastic myeloma (PBM) is a rare, aggressive subtype of multiple myeloma (MM) with poor prognosis. On the other hand, plasmablastic lymphoma (PBL) is also an aggressive large B-cell lymphoma with plasmacytic phenotype, which is generally treated with anti-lymphoma drugs. Importantly, PBM is difficult to distinguish from PBL, because clinical features of both diseases closely overlap. We report two cases of PBM accompanied by apparent extramedullary lesions. Case Case1: A 38-year-old female complained of fatigue. She presented pancytopenia, splenomegaly, the soft tissue lesion over the chest wall and multiple osteolytic lesions. Initially, pathology of the soft tissue established a diagnosis of PBL. She received two cycles of EPOCH, leading to considerable improvement. She then received daratumumab (Dara) and Lenalidomide (Len), achieving remission for two years. Case2: A 60-year-old-male was evaluated for multiple tumors of pancreas and retroperitoneum. Biopsy of pancreatic tumor identified plasmacytoid cells, whereas biopsy of bone marrow showed no evidence of plasma cells. Therefore, he was initially diagnosed as multiple plasmacytoma, and received 3 cycles of chemotherapy with Bortezomib (Bor), lenalidomide, and dexamethasone, but in vain. Once Bor was replaced to Dara, he rapidly developed panperitonitis and ascites filled with plasmablasts, and eventually died of multiple organ failure. Conclusion As there has been no standard care for PBM, our cases raise a possibility that combination therapy of anti-myeloma and anti-lymphoma drugs from the first may provide good outcome. In addition, Ki-67 proliferation index is an useful finding to diagnose as PBM. Keywords plasmablastic myeloma (PBM), plasmablastic lymphoma (PBL), Ki-67