The results are given of a 4-year prospective study regarding the occurrence, clinical pattern, and treatment of sinus arrest in patients with acute myocardial infarction. The arrhythmia was observed in 32 of 1665 patients. The detection rate increased from below 1 per cent in the first year to 5 per cent in the last due to better facilities for monitoring. All patients except one had inferior infarction. Paroxysmal atrial fibrillation was observed in 12, corresponding to a frequency approximately similar to that previously reported in patients with sinus arrest unrelated to myocardial infarction. Syncope due to sinus arrest occurred in 7 and hypotension in 19 patients. Small doses of isoprenaline given intravenously were usually effective in restoring normal heart rate, but pacemaker therapy was necessary in 3 patients.