We describe the reversal of obstructive sleep apnea with a 0.5 L increase in the functional residual capacity (FRC) in a patient with sleep apnea syndrome. The patient had been treated with medroxyprogesterone acetate for 8 months. The increase in FRC was obtained by applying a constant negative extrathoracic pressure (NEP) with a poncho-type respirator. With pulmonary inflation, there was a dramatic decrease in the apnea index and the percent apnea time, and an improvement in sleep architecture. At all sleep stages, the desaturation duration was shorter with NEP. The exact mechanisms by which pulmonary expansion improved sleep apnea in this patient remain unclear; lung volume dependence of upper airway patency and the improvements in apnea-induced desaturation may be contributing factors. Our observation illustrates that lung volumes may be an important factor in the pathophysiology of obstructive sleep apnea, especially in the apnea onset and in the apneic-induced desaturation.