The presence of antibodies to collagens type I, II, III, VI, IX, and XI was studied in patients with otosclerosis, using enzyme-linked immunosorbent assays. Levels of antibodies to collagens type II and IX were significantly higher in these patients as compared to sex- and age-matched control subjects, whereas no differences were found between the levels of antibodies to collagens type I, III, VI, and XI. These observations for the first time document the presence of autoantibodies against a minor collagen type IX in patients with otosclerosis and support a possible role for collagen autoimmunity in the etiology of otosclerosis.