None of the medical treatments of hypertrophic cardiomyopathy is perfect. In the present study conducted on 11 patients with hypertrophic cardiomyopathy in whom the usual treatments were either ineffective or badly tolerated, the haemodynamic effects of propafenone administered intravenously were investigated. The drug was injected centrally in doses of 2 mg/kg over 10 minutes, then by continuous intravenous infusion of 1.5 mg/min during 30 minutes. Various parameters were recorded before and after propafenone treatment by right and left cardiac catheterization. This anti-arrhythmic drug, which has beta-blocking and amiodarone-like properties, reduced left intraventricular obstruction but had no beneficial effect on diastolic function. The baseline and induced left intraventricular gradients were reduced from 30.4 to 17.7 mmHg and from 74 to 43 mmHg respectively. Diastolic function values showed a fall in dp/dt min from 1470 to 1307 mm/sec and an increase in T value from 0.066 to 0.084. The use of propafenone in hypertrophic cardiomyopathy must be accurately determined by long-term oral studies.