Waldenströms macroglobulinemia which was manifested by muscle pain and anemia. The female patient had suffered from back pain for about 3 years before she came to our clinic. In the last year pain in the muscles of the upper and lower extremities developed in addition to back pain. This led to the suspicion of polymyositis. However this was not confirmed by a special examination. The patient was diagnosed with clearly established infiltration of lympho-plasmacytic lymphoma and 10.8 g/l of type IgM monoclonal immunoglobulin in the bone marrow. Serum myoglobin levels and serum CK activity were repeatedly significantly increased. Therefore the treatment with anti-CD20 monoclonal antibody (Mabthera) 375 mg/m2 i. v. was started, administered once a month, with cyclophosphamide 500 mg/m2 i. v. on days 1 and 15 of a 28-day cycle, and dexamethasone 20 mg from 1st through to 4th days and 15th through to 18th days of the treatment cycle. There were 8 cycles planned. Already after a 5th cycle, the disappearance of monoclonal immunoglobulin (negative immunofixation), normalisation of myoglobin and CK values and significant relief from muscle pain were achieved. The hemoglobin concentrations before treatment were significantly reduced, while they were normalised after treatment. After 5 cycles, the complete remission of Waldenströms disease was reached according to biochemical parameters, and normalisation of the serum myoglobin and creatine kinase levels was achieved.