To define the prevalence and clinical significance of exercise-induced ST segment elevation during predischarge treadmill testing after uncomplicated acute myocardial infarction confirmed by serum MB creatine kinase (CK) activity, 241 consecutive patients were prospectively investigated with quantitative exercise thallium-201 scintigraphy, rest radionuclide ventriculography and coronary angiography at 10 +/- 3 days. All patients received customary care, and in none was thrombolytic therapy or emergency coronary angioplasty employed. Eighty-two patients (34%) had exercise-induced ST segment elevation of greater than or equal to 1 mm above rest baseline. These patients were similar to the 159 patients without this finding with respect to history of prior infarction, the Norris coronary prognostic index, exercise duration, metabolic equivalents (METs) achieved and peak heart rate-blood pressure product. The frequency of inducible myocardial ischemia and extent of angiographic coronary disease was also comparable in the two groups. Findings associated with larger infarct size and transmural extent of infarction were more common in patients with exercise-induced ST segment elevation than in those without, including higher peak CK values (1,235 +/- 1,037 versus 942 +/- 915 mumol/min per liter, p less than 0.026), lower left ventricular ejection fraction (43 +/- 12 versus 51 +/- 10%, p less than 0.001), a higher prevalence of pathologic Q waves in greater than or equal to 2 contiguous infarct-related leads (80 versus 55%, p less than 0.001), more persistent thallium-201 defects (2.2 +/- 1.1 versus 1.4 +/- 1.1, p less than 0.001), abnormally increased lung uptake of thallium (33 versus 18%, p less than 0.01) and a greater number of akinetic or dyskinetic segments (3.2 +/- 2.5 versus 1.4 +/- 1.9, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)