The effect of propranolol on precordial ST-segment elevation was studied in 24 patients with acute anterior myocardial infarction. The electrocardiographic response to the drug was correlated with the early angiographic appearance of the left anterior descending coronary artery (LAD). After a 30-minute observation period, intravenous propranolol (average dose 3.5 +/- 2.2 mg) was given a mean of 2.8 +/- 1.9 hours after the onset of persistent chest pain. Coronary angiography was performed 3.6 +/- 2.0 hours after the onset of symptoms. Patients were classified into 2 groups according to the angiographic findings. Group A consisted of 7 patients with a stenotic but patent LAD and 1 patient with excellent collateral blood flow to that area. Group B consisted of 16 patients with a completely occluded LAD and poor or absent collateral blood flow. Patients in group A showed a mean reduction in precordial ST-segment elevation of 77 +/- 18% and patients in group B showed a mean reduction of 13 +/- 14% (p less than 0.005). Left ventricular ejection fraction at discharge was 0.6 +/- 0.07 in group A and 0.37 +/- 0.08 in group B (p less than 0.001). Thus, the electrocardiographic response to intravenous propranolol given early in the course of acute anterior myocardial infarction predicts the presence of blood flow to the infarcting zone. The combination of residual blood flow and reduction of ST-segment elevation secondary to propranolol is associated with preservation of ventricular function.