ECGs obtained on arrival at the hospital from 277 patients with acute myocardial infarction were analyzed retrospectively for PR displacements, which were classified as major or minor criteria for atrial infarction and related to the later occurrence of supraventricular arrhythmia in the hospital. Major criteria were (1) PR segment elevation greater than 0.5 mm in leads V5 and V6 with reciprocal PR segment depression in leads V1 and V2, (2) PR segment elevation greater than 0.5 mm in lead I with reciprocal PR segment depression in leads II and III, and (3) PR segment depression greater than 1.5 mm in precordial leads and greater than 1.2 mm in leads I, II, and III. Abnormal P waves were classified as minor criteria. Major and minor criteria were found in 15 (5.4%) and 19 (6.9%) patients, respectively. Eight (53.3%) patients with major and six (31.6%) with minor criteria had supraventricular arrhythmias, giving odds ratios of 9.9 and 3.7, respectively. Enzyme-estimated infarct size, the occurrence of heart failure, and mortality rates did not differ in patients with or without major criteria for atrial infarction. We conclude that the occurrence of PR segment displacements on the admission ECG may predict the risk of developing supraventricular arrhythmias during hospitalization for myocardial infarction.