The prevalence of agalactosyl N-linked oligosaccharides on serum IgG was determined for patients with juvenile onset and with adult rheumatoid arthritis. A significant difference in the prevalence of these structures from age matched controls was found in both types of arthritis. In patients with adult onset rheumatoid arthritis, the results showed a strong correlation between the prevalence of IgG-associated agalactosyl oligosaccharides and disease activity. A correlation between disease activity and agalactosyl structures was also seen in a retrospective analysis of serial IgG samples from patients with juvenile onset disease. The finding that childhood onset arthritis and adult rheumatoid arthritis share a defect of glycosylation of serum IgG suggests that there may be a greater similarity between these two varieties of rheumatoid arthritis than has been hitherto considered. The observation that the incidence of agalactosyl oligosaccharides on IgG fluctuates with disease activity provides indirect evidence for a seminal role for this change of glycosylation in the inflammatory process which, in rheumatoid arthritis, is focused on the synovial tissues and results in bone erosions and joint destruction.