Antibodies to the collagen-like region of C1q have been described in patients with SLE and rheumatoid vasculitis. In this study the prevalence of both IgG and IgA C1qAb was assessed in serum samples of 385 patients with different systemic and renal diseases. The results demonstrate that the prevalence of IgG and IgA C1qAb is not restricted to the diseases in which they were originally described. C1qAb can also be demonstrated in patients with MCTD, Felty's syndrome, ankylosing spondylitis, polyarteritis nodosa, mixed cryoglobulinaemia, membranoproliferative glomerulonephritis, glomerulosclerosis, and patients with anti-glomerular basement membrane nephritis. The widespread occurrence of C1qAb of both immunoglobulin classes in systemic and renal diseases may provide insight into the mechanisms that lead to C1qAb formation.