OBJECTIVE Although many clinical investigations have found a relationship between hearing loss and diabetes mellitus, the pathophysiology of this effect remains controversial. To date, the mechanisms of hearing loss in diabetic patients have been explained in terms of microangiopathy, neuropathy and encephalopathy. However, many reports indicate that some diabetic complications are associated with oxidative stress related to the diabetes itself. In the present study, we hypothesised that oxidative stress may be a cause of hearing loss in diabetic patients. METHODS The study group comprised non-insulin dependent diabetic patients with no signs of microangiopathy or peripheral neuropathy. The control group comprised sex-, age- and body weight matched, non-diabetic subjects. Auditory function was evaluated using pure tone audiometry and tympanometry. Subjects with normal hearing and sensorineural hearing loss were included in the study, whereas subjects with conductive hearing loss were excluded. Both the study group (n = 63) and the control group (n = 37) were divided into subgroups based on the presence and absence of hearing loss. Oxidative stress was evaluated by measuring serum indicators of protein oxidation and lipid peroxidation, serum levels of nitric oxide and various non-enzymatic antioxidants, and the activity of various enzymatic antioxidants. RESULTS The non-insulin dependent diabetic patients had significantly higher serum levels of protein oxidation products, nitric oxide, enzymatic antioxidant activity (i.e. glutathione peroxidase and superoxide dismutase), compared with the control group (p < 0.05). When we compared the groups in relation to the presence of hearing loss, the nitric oxide level was significantly increased in the diabetic group with good hearing, compared with diabetic patients with hearing loss (p = 0.014). In the diabetic group, a clear, negative correlation was observed between serum levels of nitric oxide and vitamins C and E, and hearing impairment (r = -0.395, r = -0.318, r = -0.500, respectively). There was also a positive correlation between serum vitamin C concentrations and hearing levels in the control group (r = 0.417). CONCLUSIONS These results suggest that oxidative stress may play an important role in hearing impairment in diabetic patients. In this process, increased protein oxidation appears to be more important than lipid peroxidation. Nitric oxide may have a protective effect on hearing, as may some nonenzymatic antioxidants such as vitamin C and E.