Six patients treated with a combination of amiodarone and class I antiarrhythmic agents for a minor arrhythmia developed atypical ventricular tachycardia "en torsades de pointe". All patients had QT-interval prolongation in the ECG. Combined administration of quinidine and amiodarone in a normal volunteer resulted in an increase in plasma quinidine concentration and in QT prolongation, thus confirming the clinical observation of a clinically relevant interaction between the two drugs.