The effects of body position (supine, right and left lateral decubitus) on gas exchange were evaluated in a patient with agenesis of the left pulmonary artery, subjected to mechanical ventilation because of adult respiratory distress syndrome. Arterial blood gases were measured 60 min after each of the three positions was attained and showed that lying on the left side (i.e. the side of the impaired lung) produced higher arterial oxygen pressure (68.5 ± 1.5 mm Hg) than lying on the opposite side (50.0 ± 1.2 mm Hg); (p < 0.05). No significant differences in mean arterial carbon dioxide pressure were found. In contrast to unilateral lung diseases affecting the parenchyma, the position improving arterial oxygenation was the one in which the healthy lung was uppermost, in our patient with a unilateral perfusion defect.