The sugar chains of serum immunoglobulin (Ig) G, A, and M in rheumatoid arthritis (RA) patients (n=48) with elevated and normal C-reactive protein (CRP) were compared with those in age-matched healthy control subjects (n=17). The levels of agalactosylated sugar chains (GO) of serum IgG were increased significantly (p<0.01) in RA patients with elevated serum CRP, while those in RA patients with normal serum CRP remained similar to those in healthy subjects. The digalactosylated sugar chain levels were also decreased significantly (p<0.01) in serum IgA and IgM of active RA patients. Increases in levels of the bisected sugar chains were observed in serum IgG, IgA, and IgM from active RA patients. Furthermore, the GO contents of serum IgG in RA patients were correlated with their serum CRP levels (r=0.5165). These results indicated that disturbed glycosylation of Ig molecules occurs commonly in patients with active RA.