Summary:Atrioventricular block (A‐V block) was documented in 150 (13.8%) of 1083 patients with acute myocardial infarction. Those with A‐V block differed significantly from the remainder, being older, having higher peak levels of serum lactic dehydrogenase and a greater incidence of left ventricular failure and of death in hospital.These differences were due mainly to the inclusion of 90 patients with complete heart block (CHB). Among those patients whose CHB complicated anterior infarction there was a significantly greater incidence of previous infarction.Lesser grades of A‐V block and right bundle branch block (RBBB) commonly heralded the onset of CHB, which occurred more frequently in those with inferior infarction.Markers of death in those with CHB were anterior infarction, RBBB, and a slow subsidiary pacemaker with a wide ventricular complex. Pacing is recommended for all patients with CHB and for those with RBBB; the reasons for this are presented in d