Among 358 patients with rheumatic diseases, the incidence of monoclonal gammopathy of undetermined significance (MGUS) as detected by immunofixation was 4.4% (11 of 248 patients) in rheumatoid arthritis (RA), 3% (1 of 32 patients) in systemic lupus erythematosus (SLE), 6% (3 of 49 patients) in Sjögren's syndrome (SS) and 3% (1 of 29 patients) in progressive systemic sclerosis (PSS). Solid tumor was present in 4 (36%) of the 11 RA-MGUS patients. In these cases the monoclonal component could be related to a paraneoplastic syndrome rather than to rheumatic diseases. The association of rheumatic diseases, MGUS, solid tumor and immunological disorders are discussed.