A 67-year-old male was admitted to our hospital because of anemia, thrombocytopenia, and renal failure. On admission he showed splenomegaly and elevated serum LDH level. Bone marrow showed hypercellularity with massive infiltration of lymphoblastoid cells. He was diagnosed as having multiple myeloma (BJ-kappa, stage IIIB). He transiently responded to intensive chemotherapy (VAD, MP, IFN alpha) but relapsed with multiple subcutaneous tumors and pericardial effusion. This is a rare case that the myeloma cell invasion to pericardial space was diagnosed before his death. The level of interleukin 6 (IL-6) in pericardial effusion was 16382 pg/ml, and the myeloma cells obtained from the pericardial effusion responded to IL-6, which suggested that high level of IL-6 closely related to the proliferation of myeloma cells in this case.