A prospective study of 47 patients during mechanical ventilation with positive end-expiratory pressure was undertaken to evaluate changes in Pao2 following chest physiotherapy. In contrast to other studies, we found no significant change in Pao2 following chest physiotherapy (p greater than 0.05). Different technics for physiotherapy and ventilation may have accounted for the absence of hypoxemia reported by others. Possible reasons for failure to improve Pao2 were suggested. Chest X-ray before and within 24 hours of physiotherapy showed improvement in 68 percent of patients. Chest physiotherapy was most effective in the treatment of unilobar densities and produced dramatic improvement in atelectasis of acute onset.