The acute pulmonary and systemic haemodynamic response to low (0.15 mg/kg) and high (0.30 mg/kg) doses of intravenous hydralazine was evaluated in 26 consecutive patients with severe pulmonary hypertension due to cor pulmonale (nine patients), primary pulmonary hypertension (11 patients), or pulmonary embolism (six patients). Hydralazine did not cause a significant change in pulmonary arterial resistance or pressure in any group but produced a significant reduction in systemic resistance, which correlated with plasma concentration, and a significant increase in pulmonary blood flow index in all groups. Ten patients who experienced a reduction in pulmonary arterial resistance of at least 5 U X m2 after administration of hydralazine had higher initial values for pulmonary arterial resistance and systemic resistance and a lower pulmonary blood flow index than those who did not respond. Maintenance oral hydralazine treatment during nine to 36 months of follow up did not seem to affect symptoms or mortality. These results indicate that hydralazine has limited value in acutely reducing pulmonary arterial pressure or affecting clinical outcome in patients with pulmonary hypertension.