Recent reports have suggested an association between terodiline hydrochloride and cardiac arrhythmias. We report 4 patients presenting over a six month period who developed polymorphic ventricular tachycardia (polymorphic VT) while receiving treatment with this agent. In each case there was prolongation of QT interval on electrocardiogram (ECG). Two patients had hypokalaemia associated with diuretic therapy. In the 3 cases in which follow-up ECG was available, QT interval returned to normal after discontinuation of terodiline. In order to define the effects of terodiline on corrected QT interval (QTc) and heart rate in the elderly, a prospective study was performed in 8 elderly in-patients treated with terodiline for urinary incontinence. After 7 days treatment with terodiline 12.5 mg twice daily, there was a significant increase in QT by a mean of 29 ms, QTc by 15 ms and a decrease in resting heart rate by a mean of 6.7 beats.min-1. Terodiline increases QTc and reduces resting heart rate in elderly patients. Both these effects may be associated with polymorphic VT, a potentially life threatening arrhythmia. This drug should be avoided in patients with other known risk factors for polymorphic VT, particularly hypokalaemia and cardiac disease.