Arrhythmias and conduction disorders were studied in 110 patients with posteroinferior myocardial infarction with right ventricular involvement. All the patients were hospitalized in the coronary care unit, and were compared with another 110 patients with posteroinferior myocardial infarction without extension to the right ventricle. 99% of the patients with right ventricular infarction and 96.3% of the patients with isolated inferior infarction had some type of arrhythmia. The disorders of automatism were similar in both groups (90% vs 91%, respectively). The conduction disturbances were observed in 68% of the infarctions extended to the right ventricle and in 20% of the isolated left ventricular infarctions (p greater than 0.01). A-V block occurred in 52% of the infarctions with right ventricular involvement and only in 10.9% of the control group. Intraventricular conduction disorders also were more frequent in right ventricular infarction (24.5% vs 10.9%) (p less than 0.02), especially the RBBB (18.2% vs 6.4%). Ventricular fibrillation was observed in 5.5% and 0.9%, and polymorphic ventricular tachycardia (torsades de pointes) in 12.7% and 1.9% respectively. In 62 patients with right ventricular infarction it was necessary to implant a pacemaker as compared to 12 patients in the control group. Mortality was higher in the patients with inferior infarction extended to the right ventricle (23 patients vs 2 patients). None of the deaths were due to arrhythmias. It can be concluded that conduction disorders and the number of pacemaker implants are more common in the infarctions with right ventricular involvement due to more severe damage to the conduction system.