Obesity hypoventilation syndrome is an underdiagnosed pathology, whose prevalence is increasing due to the progressively higher prevalence of obesity in the general population. Early detection allows early management and lowers the risk of acute exacerbation, hospitalization and mortality. The diagnosis is done by excluding other, pulmonary or extra pulmonary, reasons of hypercapnia; a nocturnal polygraphy is mandatory to diagnose an associated obstructive sleep apnea syndrome. The pneumological treatment is a ventilatory support by CPAP or biPAP (bilevel Positive Airways Pressure).