To determine the diagnostic value of R wave amplitude changes occurring during exercise testing after myocardial infarction, exercise ECG's and coronary angiograms were reviewed in 76 postinfarction patients and in 40 patients with normal coronary arteries. During exercise, an increase in R wave amplitude (mean: + 2.7 +/- 1.3 mm) was observed in the postinfarction patients, significantly different (P less than 0.001) from the decrease (mean: - 2.6 +/- 1.1 mm) observed in the group with normal coronary arteries. Although this change increased with the number of diseased coronary arteries, the difference between 1-vessel and multi- or 3-vessel disease was not significant. Extension of infarct size from one to more akinetic segments on the left ventricular angiogram was associated with a significant (P less than 0.001) increase of the R wave amplitude change during exercise (mean: + 1.6 +/- 1.1 vs 3.3 +/- 1.3 mm). It is concluded that the abnormal increase in R wave amplitude observed during exercise testing after myocardial infarction is more strongly related to infarct size then to the number of diseased coronary arteries. Furthermore exercise induced R wave amplitude changes have no diagnostic value in the prediction of multi- or 3-vessel disease in postinfarction patients.