BACKGROUND: Obstructive sleep apnea syndrome (OSAS) is associated with cardiac arrhythmia due to autonomic dysfunction. The aim of this study was to elucidate the effect of OSAS on the QT interval and the efficacy of continuous positive airway pressure (CPAP) treatment on a prolonged QT interval. METHODS: From March 2007 to January 2008, 13 patients diagnosed as OSAS received CPAP treatment. Ten age- and sex-matched healthy controls were enrolled, and nighttime polysomnography was performed. We analyzed the full-night electrocardiogram.which is one of the elements of nighttime polysomnography.in all patients and controls. RESULTS: In the patient group, the QTc interval was 0.380+/-0.45 ms (mean+/-SD) before CPAP treatment and 0.368+/-0.32 ms during CPAP treatment. In the control group, the QTc interval was 0.348+/-0.26 ms. The QTc interval did not differ between OSAS patients and controls (p=0.143), but it did significantly shorten between before and after CPAP treatment (p=0.047). CONCLUSIONS: This study suggests that CPAP treatment can improve QTc prolongation and thereby prevent future ventricular arrhythmia.