OBJECTIVE To assess the diagnostic value of treadmill submaximal exercise testing (TSET), early after myocardial infarction (MI) under medical treatment, evaluating the ST changes predominantly. METHODS 65 patients, 62 males and 3 females, mean age 52 years with demonstrated MI under clinical, electrocardiographic and enzymatic criteria were evaluated with TSET before hospital discharge. Thrombolytic therapy in acute phase of MI was done in 45 of the patients. All patients underwent angiographic studies with left ventriculography and selective coronariography. RESULTS Of all TSET variables considered (Treadmill time, ST segment, presence of chest pain, systolic pressure, double product and presence of ventricular arrythmias) the single most important predictor for high-risk groups like left main disease and three vessels disease, was ST changes alone or associated with exercise precordial pain, with a sensitivity of 100% to left main and 83% to three vessels disease. In the thrombolytic group we did not find any differences related with reperfusion. CONCLUSIONS TSET is a useful, noninvasive, and safe method for risk stratification after MI mainly when ST segment changes are considered alone or with stress precordial pain, predicting high risk groups, namely left main or three vessels disease patients.