Resting ECGs were recorded in 29 patients with angina pectoris before, during and after treatment with prenylamine 180 mg daily. The QT interval became significantly prolonged after one week of treatment. The prolongation persisted as long as therapy was continued, which was up to 6 months. After withdrawal of treatment the QT interval returned to normal within 2 weeks. In this study no serious problems were encountered by those patients in whom the QT interval was prolonged.