A technique for differential lung ventilation via a Carlens tube with inspiratory retardation and the application of separate levels of PEEP to each lung are described in the treatment of a patient with severe unilateral pneumonia. In addition to isolating the diseased lung, this technique permits modification of the distribution of ventilation between the two lungs to improve the ventilation-perfusion ratio and enhance arterial oxygenation. Its clinical application is influenced by the flow characteristics of available mechanical ventilators and by the limitations of the currently available double-lumen endobronchial tubes.