Background: The aim of this study was to investigate the differences between pre- and post-operative hearing thresholds, measured by high frequency audiometry, in patients undergoing open heart surgery. Materials and methods: Pre- and post-operative audiometric assessments were performed in 20 patients undergoing open heart surgery. Pure tone audiometry testing was performed at 500, 1000, 2000, 3000, 4000, 6000, 8000, 10 000, 12 000 and 14 000 Hz. We also evaluated: patients' clinical parameters (i.e. age, sex, diabetes mellitus, hypertension, hypercholesterolaemia, history of myocardial infarction, and whether undergoing coronary artery bypass surgery or valve surgery); various operative details (operative temperatures, cardiopulmonary bypass time and cross-clamp time); and post-operative clinical progress and its effect on hearing loss. Results: Patients' pre- and post-operative pure tone audiometric results were significantly different at some frequencies (p < 0.05). In addition, there was a significant impact of hypertension, hypercholesterolaemia, history of myocardial infarction and cross-clamp time. Conclusions: This study shows that open heart surgery using cardiopulmonary bypass can lead to significant post-operative changes in hearing levels at some frequencies. Some additional patient parameters may influence this process. It seems possible that the risk of such hearing loss could be reduced; further studies may be able to define the significance of patients' concomitant disorders. © 2007 JLO (1984) Limited.