We assessed the impact of treatment with nasal continuous positive airway pressure (nCPAP) on the quality of life of 19 patients with obstructive sleep apnea (OSA). We measured the utility for the patients' health states before and with treatment using the standard gamble approach. The study group had an average age of 57 years and had been on treatment for a mean of 9.5 months. For all the patients, the polysomnographic indicators of OSA disease severity improved markedly with treatment. For nine of the 12 symptoms most commonly associated with OSA, the patients reported improvement during treatment. The mean utility and the standard deviation obtained with the standard gamble method were 0.87 +/- 0.17 on treatment and 0.63 +/- 0.29 pretreatment. The difference in utility between treatment and pretreatment health states was combined with the life expectancy of each patient to generate the number of quality adjusted life years (QALYs) considered equivalent to the impact of treatment. This resulted in an average gain of 5.4 QALYs. When we related this impact to the cost of treatment, we obtained a cost-utility ratio between Can $3,397 and Can $9,792 per QALY added. These costs are relatively small when compared to the cost per QALY for many other clinical interventions. Hence, nCPAP clearly offers the prospect for a well-tolerated therapy with a very favorable cost-utility ratio.