To determine the feasibility and predictive value of early exercise testing 72 hours after acute myocardial infarction, 109 consecutive patients who received reperfusion therapy were prospectively evaluated. In the group studied, in 87 (80%) the course was uncomplicated 3 days after admission, as defined by a lack of congestive heart failure, arrhythmias and angina, and 53 patients (49%) performed heart rate-limited (140 beats/min) treadmill exercise. These patients exercised for 7.9 +/- 3.4 minutes, achieving a heart rate of 129 +/- 11 beats/min and a systolic blood pressure of 151 +/- 27 mm Hg. The exercise test was not accompanied by any protracted ischemia, infarction or significant arrhythmias. Accompanying tomographic thallium-201 scintigraphy demonstrated a reversible perfusion defect in 14 patients (26%), no evidence for ischemia in 36 patients (69%) and an equivocal result in 3 patients (6%). Of the 14 patients with a positive exercise-thallium test result, 4 had an adverse clinical outcome of either reinfarction, postinfarction angina or ventricular tachycardia during hospital days 4 to 10; an adverse in-hospital outcome was not seen in the 40 patients with a negative exercise-thallium test result (p = 0.009). Thus, early exercise testing after acute myocardial infarction is safe in selected patients with an uncomplicated course and the test is predictive of in-hospital clinical outcomes.