BACKGROUND The obesity-hypoventilation syndrome (OHS), or alveolar hypoventilation in the obese, has been described initially as the "Pickwickian syndrome". It is defined as chronic alveolar hypoventilation (PaO2<70 mmHg, PaCO2 > or =45 mmHg) in obese patients (body mass index>30 kg/m2) who have no other respiratory disease explaining the hypoxemia-hypercapnia. BACKGROUND The large majority of obese subjects are not hypercapnic, even in case of severe obesity (>40 kg/m2). There are three principal causes, which can be associated, explaining alveolar hypoventilation in obese subjects: high cost of respiration and weakness of the respiratory muscles (probably the major cause), dysfunction of the respiratory centers with diminished chemosensitivity, long-term effects of the repeated episodes of obstructive sleep apneas observed in some patients. The role of leptin (hormone produced by adipocytes) in the pathogenesis of this syndrome, has been recently advocated. OHS is generally observed in subjects over 50 years. Its prevalence has markedly increased in recent years, probably due to the present "epidemic" of obesity. The diagnosis is often made after an episode of severe respiratory failure. Comorbidities, favored by obesity, are very frequent: systemic hypertension, left heart diseases, diabetes. CONCLUSIONS OHS must be distinguished from obstructive sleep apnea syndrome (OSAS) even if the two conditions are often associated. OSAS may be absent in certain patients with OHS (20% of the patients in our experience). On the other hand obesity may be absent in certain patients with OSAS. CONCLUSIONS Losing weight is the "ideal" treatment of OHS but in fact it cannot be obtained in most patients. Nocturnal ventilation (continuous positive airway pressure and mainly bilevel non invasive ventilation) is presently the best treatment of OHS and excellent short and long-term results on symptoms and arterial blood gases have been recently reported.