Non invasive ventilation (NIV) usually presents air leaks that may determine the result of this treatment. The clinical usefulness of analyzing (and quantifying) these leaks during the adaptation period to NIV has not been evaluated in prospective clinical studies as a predictive data of treatment effectiveness. Our hypothesis is that air leaks are correlated to a successful adaptation to NIV. And air leak magnitude may predict early failures of this treatment. And also, as we do not know if air leaks change during the adaptation period to NIV, we do not know whether early detection of air leaks plays a role in the therapeutic outcome. Our objective is to evaluate the clinical usefulness of quantifying air leaks during the adaptation period to NIV as a predictor of effectiveness of this treatment. We also want to evaluate the correlation between air leaks and clinical-functional patients' profile, and the ventilatory parameters selected and to evaluate air leaks variability during the adaptation period to NIV. Method: We will determine the air leak magnitude in twenty patients during adaptation to NIV. We will use VPAP III ventilators (ResMed, Australia), commercial nasal masks (Mirage o Ultra Mirage) and VPAP III/ResLinkTM (ResMed, North Ryde, Australia), a device that includes a monitoring system coupled with the VPAP III ventilator. This device allows to record air leaks and other ventilation parameters. We will perform four VPAP III/ResLinkTM recordings in each patient (the last night in hospital during the adaptation period, the first night at home, and one and two months later, after the initiation of NIV treatment). We will also perform a complete pulmonary function test, quality-of-life questionnaire (SF36), and a tolerance to NIV questionnaire in all patients at the onset of NIV treatment and two months later. We will evaluate which patients will need to change treatment at the end of the adaptation period to NIV. We also will analyze and compare air leaks magnitude in each of the four recordings stated above.