Halofantrine, increasingly used for treatment of Plasmodium falciparum malaria, is a normally well-tolerated amino-alcohol with very few side-effects, but torsades de pointes ventricular tachycardia due to halofantrine has been reported in a few patients with a congenital long QT interval (Romano-Ward syndrome). We performed a prospective study of the cardiac effect of halofantrine in 20 patients with 48 h ambulatory electrocardiographic (ECG) monitoring; the halofantrine levels in their serum were also determined. Minimal ECG changes were noted, with lengthening of the QT interval without clinical symptoms. This effect was dose-dependent and can be very severe in cases of pre-existing cardiopathy; it also occurs in patients without any pre-existing cardiopathy. In order to reduce the likelihood of such incidents, which are admittedly rare, we suggest performing electrocardiography on all patients before initiating treatment with halofantrine.