Positive end-expiratory pressure (PEEP) has been extensively utilized in the treatment of severe hypoxemia from noncardiogenic pulmonary edema. The usefulness of therapy with PEEP in the management of lobar atelectasis has not been previously stressed. Recently, we observed four patients with lobar atelectasis who failed to respond to the usual conservative measures of endotracheal suctioning and thoracic physiotherapy. Atelectasis was confirmed by physical examination and chest x-ray films, and three of the four patients subsequently underwent fiberoptic bronchoscopic examination. Endobronchial obstruction was not found, and despite extensive irrigation and suctioning, the atelectasis failed to resolve. Therapy with PEEP was then added, with pressures of 5 to 15 cm H2O. Serial chest x-ray films disclosed resolution of the atelectasis within 4 1/2 hours in two patients, within 14 hours in one patient, and within 24 hours in the remaining patient.