Relationship between rheumatoid arthritis and monoclonal gammopathy.

The incidence of monoclonal gammopathy (MG) developing after the onset of rheumatoid arthritis (RA) was found to be 4.1%. Comparison of 12 patients with both diseases and a group of 215 control cases of RA showed the age of onset of RA was higher when MG was going to develop. In such cases, MG was heralded by lower or diminishing serum rheumatoid factor titers. Comparison of these 12 patients with 971 control cases of MG showed the same high frequency of patients with IgM-MG as that recently reported for a series in Western France. An unusually high but not significant incidence of patients with lambda light chains was observed. The reasons for this remain to be elucidated.