Of 32 patients with acute myocardial infarction complicated by sinoatrial disease, 23 survived. All 23 had inferior infarction. During follow-up lasting 4 to 6 years only one patient developed severe chronic sinoatrial disease (sick sinus syndrome) necessitating permanent pacemaker treatment; twelve others died during this time. In 2 of them death was sudden 5 and 6 months after infarction. Atrial pacing studies in 7 of the 11 patients still alive showed no gross abnormalities. A review of 71 patients with chronic sinoatrial disease treated with a permanent pacemaker revealed only 5 with previous documented infarction. The present data suggest that sinus node dysfunction in patients surviving acute infarction is most often only temporary as is atrioventricular block. Occasionally, however, severe chronic sinoatrial disease requiring a permanent pacemaker may develop later, and this course of events is most likely to occur in those patients who had additional complications during the acute infarct.