One hundred seventeen consecutive patients with acute myocardial infarction transferred from the coronary care unit to a progressive coronary care unit were monitored by electrocardiographic radiotelemetry for a median of five days for arrhythmia detection. Eighty patients (68%) had arrhythmias; 37 patients (32%) had none. Almost all arrhythmias were detected by the sixth day of monitoring. Thirty-nine patients (33%) had premature ventricular contractions, two patients had primary ventricular tachycardia, one patient had primary ventricular fibrillation, and two patients had secondary ventricular fibrillation. An additional patient was successfully resuscitated after monitoring had been discontinued. Four (3%) of 117 patients had a potentially lethal arrhythmia treated successfully, three while being monitored. Twenty-five of the patients (21%) had 33 episodes of treatment intervention because of telemetrically detected events. Five patients (4%) died, all of unpreventable causes. No patient died because of an arrythmia. We conclude that telemetry is feasible and practical in detecting arrhythmias. It is clear that potentially fatal ventricular arrhythmias do occur following transfer from the coronary care unit and that monitoring is, therefore, desirable.