Chronic respiratory failure (CRF) develops in a minority of obese patients. Noninvasive mechanical ventilation (NIMV) is a new optional treatment for such patients. The aim of this study was to evaluate the effectiveness of NIMV in obese patients with CRF. The material of the study consisted of 34 obese patients (body mass index 47.3 ± 7.9 kg/m(2)) with CRF (PaO2 = 6.40 ± 0.93 kPa and PaCO2 = 8.67 ± 2.13 kPa) who were hypoxemic despite an optimal therapy. Thirteen patients had an overlap syndrome (OS) - chronic obstructive pulmonary disease (COPD) coexisting with obstructive sleep apnea syndrome (OSAS) and 21 patients had obesity-hypoventilation syndrome (OHS). Ventilation parameters were determined during polysomnography. The efficacy of NIMV was evaluated on the fifth day and after 1 year's home treatment. We observed a significant increase in the mean blood oxygen saturation during sleep in all patients; the increase was greater in patients with OHS (92.6 ± 1.4 %) than in patients with OS (90.4 ± 1.8 %). There was a significant improvement of diurnal PaO2 and PaCO2 on the fifth day of NIMV (mean PaO2 increase 2.1 kPa and PaCO2 decrease 0.9 kPa) and also after 1 year of home NIMV (mean PaO2 increase 1.9 kPa and PaCO2 decrease 2.4 kPa). Only one patient stopped treatment because of lack of tolerance during the observation period (1-3 years). In conclusion, NIMV is an effective and well tolerated treatment option in obese patients with CRF resulting in a rapid relief of respiratory disorders during sleep and a gradual, long-term improvement of gas exchange during the day, particularly in patients with OHS.