 From the Cardiologist Department of eight university hospitals CHF patients were referred to the Sleep Units with the sole condition of having a LVEF 45. After an interview and some clinical explorations, 127 out of 245 fulfilled the rigid inclusion and exclusion criteria in order to have CHF subjects without obvious comorbidity see flowchart of figure 1. These subjects went to full polysomnography PSG. Fifty seven per cent of them n: 73 had an apnea hypopnea index AHI 10. Forty three per cent n:54 had an AHI 10. Only 17 of the patients had CSA. The selection and inclusion criteria used for recruitment were the following: 1 referral of patients with chronic heart failure to the sleep laboratory; 2 diagnosis of heart failure with at least one episode of cardiac failure; 3 LVEF less than 45 using radionuclide ventriculography; 4 clinical stability for at least one month prior to inclusion; 5 optimum treatment with diuretics andor beta-blockers andor digoxin andor ACE inhibitors according to tolerance 6 no change in treatment for one month prior to inclusion; and 7 an Apnea-Hypopnea Index AHI greater than 10 measured by conventional polysomnography. The exclusion criteria were as follows: 1 patients that had a previous diagnosis of OSA or had been exposed to CPAP therapy. 2 uncontrolled arterial hypertension; 3 valvular or congenital cardiopathy, unstable angina, acute myocardial infarction or cardiac surgery in the three months prior to enrolment; 4 severe somnolence defined as severe sleepiness in situations of activity; 5 present or past medical history of clinically significant renal, liver or pulmonary disease; 6 untreated hypothyroidism; 7 clinically significant kyphoscoliosis; 8 morbid obesity with a body mass index BMI 41 Kgm2; and 9 concomitant use of the following drugs: morphine, hypnotics and sedatives, theophylline, acetazolamide or home oxygen therapy. Informed written consent was obtained from all subjects. This study was approved by the Human Ethics Committee of the 8 centres. 