Background There is increasing evidence that both obstructive and central sleep apnea contribute to the progression and prognosis in patients with chronic heart failure (CHF). In the main study of nocturnal home oxygen therapy (HOT) in patients with central sleep apnea because of CHF (CHF-HOT), significant improvements in oxygen desaturation index, apnea - hypopnea index, left ventricular ejection fraction, and specific activity scale were reported following 12 weeks of nocturnal HOT in these patients. Methods and Results The present study is designed to further evaluate the clinical efficacy and cost - benefit of nocturnal HOT according to the results of a follow-up survey on changes in frequencies of hospitalization, emergency visits, and regular outpatient visits by 53 patients undergoing nocturnal HOT for more than 6 month periods. Medical costs were estimated from the DPC-MDC5 charge for hospitalization because of worsening heart failure (HF), and from the standard model case estimation for emergency and regular outpatient visits for HF. To reveal the time-saving benefit following nocturnal HOT, the influence on estimated days spent for hospital-care was also analyzed. The present study revealed significant reduction in frequencies and length of hospitalization (2.1 to 0.5 times/year, 38.7 to 34.6 days, medical cost: -2,686,267 yen), emergency visit (2.5 to 0.7 times/year, -15,984 yen), and regular outpatient visit (17.7 to 12.6 times/year, -6,324 yen) as compared with those before the induction of nocturnal HOT, which resulted in a total medical cost-reduction of 1,854,175 yen/patient/year, even with the additional charge for nocturnal HOT (854,400 yen/patient/year). Furthermore, nocturnal HOT produced a remarkable decline in estimated days spent for hospital-care (88.2 to 21.2 days/patient/year). Conclusion The present analysis calculated a remarkable cost-benefit (1,854,175 yen/patient/year) from the reduction in hospitalization and emergency visits, and also time-saving benefits from an increase in expected days free from hospital-care (67 days/patient/year). (Circ J 2007; 71: 1738 - 1743)