This study was done to evaluate whether cardiac dysfunction or abnormal measurements on cardiopulmonary exercise testing (CPET) can be improved after 2 months of nasal CPAP treatment. Twenty patients with moderate or severe OSAS received nasal CPAP treatment. All subjects also underwent blood pressure, simple spirometric, and arterial blood gas (ABG) measurements; cardiac evaluation by radionuclide scanning and CPET; and an overnight polysomnography sleep study before and after nasal CPAP treatment. No difference in left ventricular ejection fraction (LVEF) was found after 2 months of nasal CPAP treatment, but higher right ventricular ejection fraction (RVEF), VO2peak, VO2peak/kg and workpeak were observed. After 2 months of nasal CPAP treatment, these patients had a lower breathing reserve and a greater increase in anaerobic threshold and oxygen pulse. Moderate to severe OSAS patients before nasal CPAP treatment had abnormal CPET as reflected by lower RVEF, VO2peak/kg, workpeak, anaerobic threshold and oxygen pulse. These abnormalities can be improved after 2 months of nasal CPAP treatment.