Autoantibodies against C1q have been described in many immune-complex diseases including hypocomplementaemic urticarial vasculitis and systemic lupus erythematosus (SLE). No study has focused on the role of anti-C1q antibodies in hepatitis C virus (HCV) infection. The aim of this study was (i) to evaluate the prevalence of anti-C1q antibodies in HCV infection; and (ii) to analyse the association of anti-C1q antibodies with clinical and biological features of HCV–mixed cryoglobulinaemia (MC) vasculitis. We searched for anti-C1q antibodies using an enzyme-linked immunosorbent assay (ELISA) test in 111 HCV patients (75 had cryoglobulin and 23 systemic vasculitis), 60 SLE patients and 109 blood donors. Anti-C1q antibodies were detected in 26% of HCV patients compared to 10% of healthy donors (P < 0·01), and 38% in patients with SLE. Although there was a higher prevalence of anti-C1q antibodies among HCV patients with type III cryoglobulin (50%, P < 0·01), the overall prevalence of anti-C1q antibodies was similar in HCV patients being cryoglobulin-positive or cryoglobulin-negative (26%versus 25%, P = 0·98). A significant association was found between anti-C1q antibodies and low C4 fraction of complement (P < 0·05). No association was found between anti-C1q antibodies and HCV genotype, severity of liver disease or with specific clinical signs of HCV–MC vasculitis. This study shows an increased prevalence of anti-C1q antibodies in HCV-infected patients. Anti-C1q antibodies were associated with low C4 levels. No association was found between anti-C1q antibodies and HCV–MC vasculitis, nor between anti-C1q antibodies and cryoglobulinaemia.