Analysis of the course of 71 patients with acute myocardial infarction complicated by bundle branch block (BBB) confirms a high incidence of atrioventricular (A-V) block (42 per cent) and severe pump failure (35 per cent) in these patients. Hospital mortality was not correlated with BBB per se, but rather with the associated development of second or third degree A-V block (57 per cent with A-V block vs. 12 per cent without A-V block; p less than .0005) or severe pump failure (35 per cent with vs. 11 per cent without severe pump failure; p less than .001). However, late mortality was high and not significantly different among those surviving hospitalization whether transient A-V block was present or absent. Eight of 11 late deaths were sudden. Temporary pacing could not be shown to alter hospital survival statistically, but made the onset of complete heart block a hemodynamically smooth and clinically undetectable event in several patients who later survived. The place of permanent pacing in these patients cannot be clearly determined on the basis of this study or in the available literature. More data obtained either by pooling the experience of several centers or from a prospective randomized study are needed to determine the indications for permanent pacemakers.