Darpo B, Allared M, Edvardsson N. Karolinska, Stockholm. Int J Cardiol 1996;53:311-3. This case-report describes a patient who developed a torsades de pointes tachycardia after infusion of almokalant, a selective class III antiarrhythmic agent. The patient was studied with transesophageal atrial stimulation because of Wolff-Parkinson-White syndrome. After a base-line procedure during which an orthodromic tachycardia was induced and pace-terminated, almokalant was given intravenously. The corrected QT interval was markedly prolonged despite similar plasma concentration compared to the rest of the studied patients. During the continued pacing protocol several episodes of non-sustained ventricular tachycardia was observed after pacing induced pauses. A sustained orthodromic tachycardia with left bundle branch morphology was induced, and another almokalant infusion was given. At a plasma concentration of approximately 252 nmol/l the corrected QT interval was further prolonged to 680 ms and the patient developed a torsades de pointes tachycardia after a pacing induced pause. The tachycardia degenerated into ventricular fibrillation that required immediate defibrillation. One week later the patient underwent ablation of the accessory pathway. The QT interval was in the absence of preexcitation normal, and programmed electrical stimulation did not reveal any ventricular arrhythmias. Further studies will have to be performed to clarify whether an early and marked QT interval prolongation, such as observed in this patient, will be useful in identifying patients prone for proarrhythmias in relation to therapy with selective class III drugs.