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OBJECTIVE: Calcium antagonists are the only oral vasodilators shown to influence mortality in primary pulmonary hypertension, but the high doses required are often poorly tolerated. Amlodipine is a novel, relatively well tolerated, calcium antagonist. It has not been previously tested in humans with pulmonary hypertension. DESIGN: Calcium antagonists are claimed to be of benefit in the 20-30% of patients who respond--that is, whose mean pulmonary artery pressure and pulmonary vascular resistance decreased by 20% after acute administration. Increasing oral doses of amlodipine (up to 40 mg) were given and haemodynamic measurements were obtained by the use of indwelling pulmonary artery catheters 12 h after each dose. SETTING: Large teaching hospital, primary referral centre. PATIENTS: Six patients (four women; age range 37-78 years) with pulmonary hypertension (one with primary pulmonary hypertension, five with thromboembolic disease. MAIN OUTCOME MEASURES: Mean pulmonary artery pressure and pulmonary vascular resistance decreased by greater than 20% in two patients, mean pulmonary artery pressure decreased by greater than 20% in one patient with a pulmonary vascular resistance reduction of 19%. Thus, two of six patients responded to amlodipine and one partially responded. RESULTS: The whole group mean (SEM) pulmonary artery pressure decreased from 47.7 (4.2) to 41.7 (4.4) mm Hg and mean pulmonary vascular resistance from 8.6 (2.1) to 7.1 (1.8) Wood units. Cardiac output rose by a mean (range) of 4% (-20.8 to+20.8), heart rate by 8.8% (-10 to +33), and systemic systolic blood pressure decreased by 12% (-29.2 to -5.8) and diastolic blood pressure by 6.8% (-28.2 to+20.0). There were no symptoms of systemic hypotension. CONCLUSION: These results show that oral amlodipine can produce acute pulmonary vasodilatation in patients with pulmonary hypertension. Further studies are required, but amlodipine may prove to be of value in the treatment of primary pulmonary hypertension.