Six patients were artificially ventilated for a focal lung disease localized to one lung in four cases and to both lower lobes in two. Despite an inspired oxygen concentration of 100% the mean PaO2 was 115 mmHg. The addition of PEEP slightly improved PaO2 in two patients but led to deterioration in four. We therefore studied the effects of posture. Patients with unilateral disease were placed in the lateral position with the healthy lung dependent. The two patients with both lower lobes involved were tilted into the Trendelenburg position. The mean PaO2 rose from 98.4 to 199.5 mmHg. Posture was maintained as long as a beneficial effect was demonstrable. The disappearance of this effect was associated either with recovery (three patients) or with the extension of the pneumonia (three patients). The improvement of gas exchange can be accounted for by the rearrangement of ventilation/perfusion relationships. This management could avoid the need for differential lung ventilation.