The extent of coronary artery disease after primary myocardial infarction is an important prognostic factor. The predictive value of exercise electrocardiography for multivessel disease was assessed by comparison with coronary angiography in a series of 100 patients. In the group of patients with primary anterior infarction (n = 48), 37.5% had positive exercise ECGs. Coronary angiography showed 62.5% single vessel disease. In multivessel disease, the sensitivity of exercise ECG was 78% and specificity 86%. The predictive value of a positive test was 78% and, for a negative test, 86%. In primary inferior infarction (n = 52), positive exercise ECGs were recorded in 48% of cases. Coronary angiography showed 48% multivessel disease. In multivessel disease, sensitivity of exercise ECG was 92% ans specificity 93%. The predictive value of a positive test was 92% and of a negative test, 93%. Therefore, exercise ECG gives a more precise prediction of the extent of coronary artery disease after primary inferior infarction than after primary anterior infarction. However, it is not an ideal guide for the choice of patient for coronary angiography because of its poor predictive value in cases of anterior infarction, and because of the high incidence of multivessel disease in patients with primary inferior infarcts. Nevertheless, it is of great prognostic value with regards to the eventual course of the coronary disease.