Thirty patients with clinically significant ventricular ectopy were treated with propafenone, a new potent antiarrhythmic agent with membrane stabilizing action. Patients had a minimum mean of greater than 30 ventricular premature beats per hour documented by continuous 48-hour ambulatory ECG recording. Twenty-five patients qualified as responders, defined as greater than 85% reduction of ventricular ectopy compared to baseline, and completed a double-blind placebo-crossover phase. Significant reduction in single ventricular ectopy per hour, paired ventricular ectopy per hour, and ventricular tachycardia beats per hour were observed. Almost total abolition of ventricular tachycardia and paired ventricular ectopy was achieved. Side effects were minimal and well tolerated. A significant prolongation of the PR interval occurred. QRS prolongation and prolongation of the corrected QT interval was observed in some patients, with new left bundle branch block developing in two patients. Long-term efficacy and safety studies will be necessary to determine the ultimate role of this new agent in the selection of antiarrhythmic therapy, but these initial results are encouraging.