End-tidal CO2 partial pressure (PETCO2) has been suggested as a noninvasive index reflecting cardiac output under constant ventilation. The aim of this study was to examine whether PETCO2 does reflect cardiac output, even during exercise, in patients with acute myocardial infarction (AMI) undergoing exercise training early after onset. Method and Results Patients aged 47-73 years were randomly assigned to either a training group (n=18) or a control group (n=18) 1 week after the onset of AMI. Those in the training group performed exercise training under supervision at the anaerobic threshold level for 2 weeks, while patients in the control group followed a conventional walking regimen. In the training group, but not in the control group, PETCO2 at the respiratory compensation point increased significantly from 39.1+/-3.5 to 41.1+/-3.7 mmHg (p<0.01). Similarly, the cardiac index at peak exercise increased only in the training group (from 6.04+/-0.98 to 7.31+/-0.97 L/min per m2, p<0.01). These 2 measurements correlated well both before and after the study period. Peak oxygen uptake and anaerobic threshold were increased only in the training group. Conclusions Aerobic exercise training early after the onset of AMI significantly increased PETCO2 during exercise, which may reflect an improvement in cardiac output during exercise in response to physical training via a decreased ventilation-perfusion mismatch.