Prader-Willi syndrome (PWS) is a genetic disorder, characterized by shorter height, severe obesity and muscular hypotonicity. In particular, sleep disordered breathing (SDB) is a well-known complication in PWS. We encountered one case of PWS, complicated by typical obesity hypoventilation syndrome. A 23-year-old woman had been given a diagnosis of PWS as age 1, therefore she was treated with growth hormone replacement therapy, and with uvulopalatopharyngoplasty (UPPP) for her narrow throat. Her weight increased greatly to 96 kg, body mass index (BMI) 51 kg/m², resulting in hypersomnolence, cyanosis, heavy snoring, and nocturnal awakening. Eventually, she was admitted because of urinary incontinuence and loss of consciousness. On admission, she had severe hypoxia plus substantial hypercapnia, and her chest X-ray film showed severe cardiomegaly with massive pleural and pericardial effusion. On polysomnography (PSG) one week later, her apnea hypopnea index (AHI) was 16 with a mean nocturnal arterial saturation of 74%, mean percutaneous PCO₂ 59 Torr, which rose to 73 Torr during REM sleep. Non-invasive positive pressure ventilation (NPPV) was initiated, and improved her condition greatly. She was discharged, but continued to recieve NPPV, and her condition has stayed improved.