The effect of body position on ventilatory function was evaluated in a patient with unilateral lung disease. The patient's pulmonary dynamics were examined in the supine, right, and left decubitus positions under conditions of positive pressure ventilation with zero end-expiratory pressure (ZEEP) and 5 cm H2O (0.9 KPa) positive end expiratory pressure (PEEP). When the patient was positioned so that the "diseased" lung was dependent, there was a marked decrease in PaO2 and increase in venous admixture when compared to the values in the supine position. These changes were relatively greater in the ZEEP, than the PEEP situation. When the "diseased* lung was not dependent, there was an increase in PaO2 and a decrease in venous admixture. This was most pronounced when PEEP was applied. Changes in body position may result in clinically significant alterations in pulmonary gas exchange, especially in patients with pre-existing pulmonary dysfunction.