Correlations between baseline hemodynamic and oximetric variables during an invasive exercise test and an improvement in peak oxygen uptake (peak VO2) after exercise training (ET) were examined in 20 patients who participated in a cardiac rehabilitation program after acute myocardial infarction (AMI). Peak VO2 significantly increased by 23 +/- 21% (p < 0.01) after ET and the improvement best correlated with the change in O2 extraction fraction ([arterial O2 content-venous O2 content]/arterial O2 content) during an exercise testbefore ET (r = -0.61, p<0.01). Exercise capacity was improved to a greater extent by ET in patients with a smaller increase in O2 extraction fraction during an exercise test before ET. Thus, O2 extraction fraction during an exercise test before ET may be a useful predictor of the improvement in exercise capacity after ET in post-AMI patients.