IgG and IgM rheumatoid factors (IgG-RF and IgM-RF), complement and three assays for immune complexes were measured in 22 patients with rheumatoid arthritis (RA) complicated by either chronic active synovitis or vasculitis. Patients with vasculitis had relatively inactive arthritis but had higher titres of rheumatoid factors, especially IgG-RF, anticomplementary activity (ACA) and lower levels of C4 than those with synovitis. Clq-binding and platelet aggregation (PA) levels were similar in both groups. Serial measurements during cytotoxic therapy showed a close temporal relationship between the clinical features of vasculitis and levels of IgG-RF, ACA and C4 both with remission and with relapse. We suggest that immune complexes containing IgG-RF which activate complement and are detected by ACA are useful markers of rheumatoid vasculitis and may be important in its pathogenesis.