Hyperglycaemia and/or oxidative stress can cause IgG to be modified by advanced glycation end products (AGE). Three patients with aggressive rheumatoid arthritis (RA) and vasculitis are described who have high titres of IgM antibodies against AGE-modified IgG (IgM anti-IgG-AGE). Diabetics and randomly selected patients with rheumatic diseases, including 50 additional RA patients, were tested for IgM and IgA anti-IgG-AGE by ELISA. AGE-modified proteins were detected using the nitroblue tetrazolium (NBT) colorimetric method. The presence of Nepsilon (carboxymethyl) lysine, an AGE modification, was detected on IgG-AGE by immunoblotting. A total of 20/41 (49%) rheumatoid factor (RF)-positive RA patients tested had IgM anti-IgG-AGE antibodies, 4/12 (33%) RF-positive systemic lupus erythematosus (SLE) patients, 3/5 RF-positive patients with primary Sjogren's syndrome (SS), and 3/5 RF-positive diabetics. All patients with RF-negative RA, SLE, SS, osteoarthritis (24), spondyloarthritis (15), adult-onset Still's disease (8), diabetes (25) and healthy controls (20) were anti-IgG-AGE negative. RF and IgM anti-IgG-AGE appeared to be a linked response. The IgM anti-IgG-AGE, along with IgG-AGE, may contribute to the pathogenesis of RA.