Multiple sclerosis (MS) has been reported in association with haematological abnormalities, including monoclonal gammopathies. We present a 54-year-old male patient with a 30-year history of MS who was admitted to our hospital for investigation of anaemia and increased erythrocyte sedimentation rate. A monoclonal IgM protein was detected by serum protein immunofixation, while bone marrow biopsy indicated a 70% infiltration by small lymphoplasmacytoid cells, in the context of a lymphoplasmacytoid immunocytoma, findings compatible to the diagnosis of Waldenström's macroglobulinemia (WM). To our knowledge, this is the first report of WM in a patient with MS. Further to the coexistence of the two diseases observed in the case presented here, there is additional evidence suggesting that the association of MS with plasma cell dyscrasias may not be coincidental.