The haemodynamic effects of an intravenous amiodarone infusion (5 mg/kg for 10 minutes) were measured in ten patients with hypertrophic cardiomyopathy (two with a left ventricular outflow gradient at rest) five, 15, and 30 minutes after drug administration. Mean (SD) pulmonary capillary wedge pressure rose significantly at five and 15 minutes (from 12.3 (6.2) mm Hg to 17.6 (9.2) and to 16.2 (8.6] with a subsequent tendency to fall to control values at 30 minutes. Mean right atrial and right ventricular end diastolic pressures increased from 3.6 (1.8) mm Hg to 7.3 (3.1) and from 6.3 (2.4) to 9.8 (3.2) mm Hg respectively at 30 minutes. The increase in filling pressures was paralleled by a decrease of left ventricular max dP/dt from 1522 (414) to 1372 (327) to 1316 (338) and to 1326 (379) five, 15, and 30 minutes after infusion. Despite this slight negative inotropic effect, cardiac index and stroke volume index were unchanged or slightly increased, possibly because of the decrease in systemic vascular resistance (from 1326 (330) dyn s cm-5/m2 to 1152 (285]. In both patients with outflow gradient the pressure gradient at rest decreased (from 110 to 65 and from 85 to 65 mm Hg) through a reduction of left ventricular systolic pressure. Thus short term intravenous infusion of amiodarone is safe in patients with hypertrophic cardiomyopathy. The main changes were a mild depression of ventricular contractility, which was well tolerated and adequately compensated for by a decrease in afterload.