Although polysomnography is indispensable in the diagnosis of sleep apnea syndrome (SAS), a common disease in Japan, it is a time-consuming procedure. We therefore assessed the clinical usefulness of a cardiopulmonary sleep study (CPSS), which monitors indices of respiratory and circulatory kinetics during sleep. Of ninety male patients (50± 11 years old, BMI: 27± 4), those with apnea indices (AI) > 5, as determined using an apnomonitor, underwent CPSS with a Night Watch System. Sixty-nine patients with indications for nasal CPAP (NCPAP) therapy underwent overnight CPSS and the optimum pressure was determined. AI was 31± 17, whereas CPSS showed an apnea-hypopnea index of 49± 24, a mean SaO₂ of 93± 3%, and a minimum SaO₂ of 71± 11%, indicating severe OSAS. The mean optimum pressure was 10± 2 cm H₂O, and the compliance was 86%, which was comparatively good. As CPSS can be performed on many patients in a short time, it may be useful for the early diagnosis and initial treatment of SAS.