A patient with Marfan's syndrome and obstructive sleep hypopnea syndrome is reported. She had complained about insomnia, tiredness and hypersomnolence lasting 2 years. A complete nightly polysomnography revealed 94 obstructive hypopneas (the hypopnea index was 12.85) and no obstructive apneas. Presumably an increased laxity of the pharyngeal wall was responsible for the phenomenon. A therapy with CPAP was started, but was not tolerated by the patient. Oxygen therapy on the other hand turned out to be effective and well tolerated.