This report deals with two patients who suffered sustained episodes of torsade de pointes ventricular tachycardia while using the novel antimalarial drug halofantrine. Both patients had congenital long QT syndrome, and their QT interval was further prolonged at the time of the event. This first electrocardiographic documentation of ventricular arrhythmias together with halofantrine's known prolonging effect on the QT interval demonstrates that the drug has the potential to induce life-threatening arrhythmias.