The usefulness and limitations of antiarrhythmic drugs in ventricular tachycardias (VT) associated with congestive heart failure remain uncertain. The purpose of this study is to evaluate the proarrhythmic effects of antiarrhythmic drugs in patients with refractory VT associated with left ventricular dysfunction using electrophysiologic study (EPS). Twenty-four patients with left ventricular dysfunction, defined by left ventricular ejection fraction (LVEF) lower than 40% using left ventriculography, were studied. The average LVEF was 29.5%. As for underlying heart disease, 14 had old myocardial infarction, 8 cases had dilated cardiomyopathy and 2 had aortic regurgitation. As a control to this group, 23 cases with underlying heart disease and LVEF higher than 40%, and 27 cases with no obvious heart disease were studied. We considered a drug to have proarrhythmic effects if 1) it de-creased by one the number of stimuli needed to induce VT, 2) induced non-sustained VT in the control study which changed to induced sustained VT, 3) the sustained VT or ventricular fibrillation was newly induced, or 4) the induced sustained VT which was stopped by pacing in the control study changed to induced VT which could not be terminated by pacing and required DC shock. Proarrhythmic effects were recognized in 17 of 24 cases with left ventricular dysfunction. Of the 67 drug trials, proarrythmic effects were seen in 26. Proarrhythmias were observed in 9 of 23 cases (39.1%) with organic heart disease associated with LVEF higher than 40%. In 12 of 69 drug trials (17.4%) proarrhythmias were observed. Of 27 cases with no obvious heart disease 10 cases (37%) had proarrhythmias. In 14 of 130 drug trials (10.8%), proarrhythmias were recognized. In conclusion, the incidence of ventricular arrhythmias on EPS in cases with low LVEF was 70.8%, and 33.8% in drug trial, higher than in cases with organic heart disease and LVEF higher than 40%. Although the relation to spontaneous occurrence is not always clear, the effects of antiarrhythmic drugs therapy on ventricular arrhythmias in patients with congestive heart failure needs careful consideration and should be an interesting topic for future research.