We investigated the presence of IgG and IgA antibodies to C1q in serum samples from 80 patients with rheumatoid arthritis (RA), 31 patients with rheumatoid vasculitis, and 80 healthy controls. IgG and IgA antibodies to C1q, as measured by enzyme-linked immunosorbent assay, were found in less than 5% of the sera from RA patients and from healthy controls. In contrast, IgG and IgA antibodies to C1q were found in 29% and 61%, respectively, of the sera from patients with rheumatoid vasculitis. The occurrence of IgA antibodies to C1q has not been previously demonstrated. These results also demonstrate that IgG antibodies to C1q do not occur exclusively in systemic lupus erythematosus patients: Sera of patients with rheumatoid vasculitis frequently contain IgG or IgA antibodies to C1q, which contribute to immune complex formation.