BACKGROUND: Utility indices are used in cost-effectiveness analyses as a measure of quality of life reflecting the patient's preference for a given health state on a scale anchored at 0 (corresponding to death) to 1 (perfect health). It is uncertain which utility instruments are most suitable for application in patients with the obstructive sleep apnea syndrome (OSA). OBJECTIVES: To compare utility indices obtained in OSA patients by various instruments. Methods: In 66 untreated OSA patients (median Epworth score 12, apnea/hypopnea index, AHI, 57/h), five different utility instruments were employed. In 34 OSA patients, changes in utility after 4 months of continuous positive airway pressure (CPAP) were retrieved from published SF-36 data. RESULTS: In 66 OSA patients, median (quartiles) utility indices were: standard gamble 0.97 (0.89; 0.99); time trade-off 0.94 (0.81; 0.99); EuroQol questionnaire (EQ-5D) 0.92 (0.83; 1.00); Euro-thermometer visual analog scale 0.80 (0.70; 0.90); SF-36 questionnaire (SF-6D) 0.75 (0.69; 0.85; p < 0.05 SF-6D and Euro-thermometer utility vs. other indices). Different utility indices were poorly correlated among each other and with AHI and Epworth scores. SF-6D utility after 4 months of CPAP had changed by 0.04 (0.02; 0.12, p = 0.026). CONCLUSIONS: Utility indices measured by different instruments vary largely and some indices reflect the impaired quality of life in OSA poorly. Interpretation of cost-effectiveness analyses should account for the utility instrument used.