Heart block was noted in 60 (35 complete and 25 second-degree) of 410 patients with acute inferior wall myocardial infarction. This group with heart block was compared to a control group of 30 patients with acute inferior wall infarction without heart block. The incidences of prior myocardial infarction and hypertension, in addition to the highest level of serum creatine phosphokinase and a maximum degree of ST-segment elevation in the inferior leads, were all greater in patients with heart block, as compared to the controls. The incidences of various complications, including dizziness and syncope, transient hypotension, cardiogenic shock, and congestive heart failure, were also higher in the group with heart block, while sinus nodal distrubances and atrial arrhythmias occurred with equal frequency. The mortality in those with heart block was 28 percent compared to 13 percent for the control. It is concluded that patients with heart block complicating acute inferior myocardial infarction have a greater amount of myocardial necrosis, a higher incidence of complications, and a higher mortality. Insertion of a temporary pacemaker should be considered when specific indications are present and not routinely.