The management of patients with unilateral pneumonia and severe hypoxemia often represents a therapeutic challenge. Mechanical ventilation with the diseased lung uppermost may improve gas exchange, but it is not devoid of adverse effects. No hemodynamic measurements have been reported in patients ventilated in this manner; therefore, whether or not the improvement in PaO2 is counterbalanced by hemodynamic deterioration remains unknown. Almitrine bismesylate is a drug that seems able to improve gas exchange in patients with chronic obstructive pulmonary disease or the adult respiratory distress syndrome. The increase in PaO2 after its administration has been attributed to an improvement in ventilation-perfusion relationships. Its use has never been reported during unilateral pneumonia with severe hypoxemia. We therefore compared its effects with those of lateral position in eight consecutive mechanically ventilated patients with unilateral pneumonia. Blood gas and hemodynamic measurements were performed both at maintenance FIO2 and at an FIO2 of 1.0. Almitrine (1 mg/kg over 1 h) had no effect on PaO2 under either FIO2 condition. Cardiac output remained unchanged, but mean pulmonary artery pressure increased from 22.5 +/- 1.2 to 26.5 +/- 1.3 mm Hg (p < 0.02). By contrast, lateral position had striking effects on PaO2, which increased from 100 +/- 14 mm Hg in supine position to 156 +/- 23 mm Hg (p < 0.01) when the abnormal lung was placed uppermost at maintenance FIO2 and from 207 +/- 21 (supine) to 300 +/- 28 mm Hg (lateral) (p < 0.01) at FIO2 1.0.(ABSTRACT TRUNCATED AT 250 WORDS)