The efficacy and safety of a single infusion of cibenzoline, a class I antiarrhythmic drug, were assessed in 86 patients presenting with a supraventricular tachyarrhythmia following heart surgery in a placebo-controlled double-blind parallel study. Cibenzoline was effective in 13 patients (30%) vs three patients (6.9%) with placebo (P < 0.05). In the non-converted patients, ventricular rate was significantly slowed by cibenzoline (P < 0.05), which also significantly increased the duration of QRS and QT intervals. Transient adverse events were seen in nine patients receiving cibenzoline: moderate hypotension, sweating, right bundle branch block. One patient with decreased left ventricular function had an increased ventricular rate and QRS duration associated with hypotension. There were no severe adverse events. These results suggest that cibenzoline is effective for the treatment of postoperative supraventricular tachyarrhythmias in patients without impairment of ventricular function.