Ascites is a rare complication of multiple myeloma. We report a case of multiple myeloma with peritoneal involvement. An 80-year-old woman was admitted to our hospital because of massive ascites. Laboratory findings included an Hb of 7.8 g/dl, IgA of 2,160 mg/dl, and CA125 of 942 IU/ml. Immunoelectropheresis analysis detected a monoclonal component of IgA lambda in serum, and of BJP lambda in urine. Bone marrow aspirtion revealed normal cellularity with 32.7% atypical plasma cells. The ascites was characterized by an exudate with numerous atypical plasma cells and elevated IgA and CA125 levels. The IL-6 level was 22 pg/ml and 79 pg/ml in serum and ascites, respectively. A diagnosis of IgA lambda multiple myeloma with peritoneal involvement was made. Chemotherapy consisting of melphalan and prednisolone reduced the serum IgA and CA125 levels markedly, and alleviated the patient's ascites.