OBJECTIVE To investigate the frequency of sensorineural hearing loss (SNHL) in patients with mixed connective tissue disease (MCTD). METHODS The study population consisted of 71 patients with MCTD (69 female; 2 male), with a mean age of 57.1 +/- 7.9 years and a mean disease duration of 14.5 +/- 8.0 years. All patients underwent audiological evaluation that included pure tone and speech audiometry. In addition, the systemic manifestations of the disease and drug therapy were recorded. All patients were tested for presence of autoantibodies. Fifty-one age-matched healthy subjects served as controls. RESULTS SNHL was found in 33 (46.4%) of the 71 patients with MCTD. There was no correlation between SNHL and age and disease duration. An association was found between Raynaud's phenomenon (p < 0.03), secondary antiphospholipid syndrome (APS) (p < 0.05), and SNHL. MCTD patients with SNHL had higher serum levels of anti-U1RNP (p < 0.05), antiendothelial cell antibodies (p < 0.001), and IgG type anticardiolipin antibodies (p < 0.0001) than patients without SNHL. Serum levels of interferon-gamma and tumor necrosis factor-alpha were increased in MCTD patients with SNHL compared to patients without SNHL. The absolute number of natural (CD4+CD25(high)FoxP+) regulatory T cells (Treg) was lower compared to patients without SNHL. CONCLUSIONS In MCTD, SNHL is a specific organ manifestation and appears frequently. We have found that pathogenic autoantibodies, decreased levels of regulatory T cells, and overexpression of proinflammatory cytokines may play a role in the pathogenesis of immune mediated inner ear disorders in MCTD.