IgA class anti-IgA antibody was sought by an immunoabsorbent technique in sera from patients with IgA nephropathy (IgA-N, n = 62), other forms of primary glomerulonephritis (PGN, n = 41), seropositive rheumatoid arthritis (RF-positive, n = 18) and normal controls (c, n = 50). IgA-N (31%) and RF-positive (56%) patients showed a significant increase in IgA anti-IgA antibody levels. The subclass of IgA anti-IgA antibody was predominantly IgA1 in both IgA-N and RF-positive patients. Size analysis revealed the dimer/monomer ratio to be significantly increased in IgA-N patients compared with that of RF-positive patients (p = 0.03). These results indicate the possible existence of the dimeric form of IgA class anti-IgA antibody in the circulation of IgA-N patients.