The effect of mechanical chest vibration on arterial blood gases was studied in 10 patients requiring oxygen therapy who had acute lung disease and an abnormal chest roentgenogram. Eight patients were using mechanical ventilators. For the duration of the study, patients were seated or in a high semi-Fowlers position. No other pharmacologic or physical therapy was given during the study period, nor was there any change in the fraction of inspired oxygen or the respirator settings. The patients ranged from 23 to 89 yr of age. After control blood gas measurements were taken, each patient received 30 min of mechanical vibration to the thorax followed immediately by tracheal suction. Blood gases were re-examined 30 min and 1 h after completion of the physiotherapy. Results showed a significant increase in PO2 at 30 min and 1 h after completion of mechanical chest vibration with no change in PCO2 or pH. This study demonstrated that external mechanical vibration of the chest is a useful therapeutic modality in the management of hypoxemia in patients with atelectasis or pneumonia. This was reflected by the increase in PO2 with no change in alveolar ventilation. We presume the better matching of ventilation to perfusion accounted for our results.