Nifedipine was given, 20 mg sublingually, to 6 patients with pulmonary hypertension secondary to advanced chronic obstructive pulmonary disease, and its effects on hemodynamics, blood gases, lung mechanics, and the distribution of ventilation-perfusion ratios (VA/Q) were investigated. Systemic vasodilation was obtained, with a reduction in mean systemic arterial pressure and in systemic vascular resistance by 16 and 36%, respectively. Cardiac index increased by 29%. Pulmonary vascular resistance decreased by 28%, without changes in pulmonary arterial mean pressure. Arterial PO2 decreased from 52 +/- 4 to 47 +/- 3 mmHg (p less than 0.001). A deterioration in VA/Q matching could be demonstrated, with a redistribution of blood flow into the lungs by a diversion of 20% of total blood flow from units with normal VA/Q, between 0.23 to 3.0, to hypoxic units with low VA/Q between 0.19 to 0.009. These changes might be explained by a partial inhibition of hypoxic pulmonary vasoconstriction.