Background: Inhibition of the renin-angiotensin system (RAS) with angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor blockers (ARBs) can suppress left ventricular (LV) remodeling after acute myocardial infarction (AMI), possibly through the modifications of matrix metalloproteinase (MMP)-9. Whether LV remodeling is suppressed in association with MMP-9 suppression in post-AMI/-percutaneous coronary intervention (PCI) patients treated with ACE inhibitor and/or ARB was examined. The presence of any differences in LV remodeling and MMP-9 levels across the groups was also investigated. Methods and Results: Sixty-five patients were initiated into each of 3 treatments; trandolapril, valsartan or a combination of both (half-dose-trandolapril plus half-dose-valsartan). Changes in MMP-9, LV end-diastolic and end-systolic volume index (LVEDVI and LVESVI) after 12 months were assessed. Overall, MMP-9 significantly decreased, although neither LVEDVI nor LVESVI increased significantly. ΔMMP-9 was significantly correlated with ΔLVEDVI (r=0.36) or ΔLVESVI (r=0.39). In comparison, across groups, it was found that MMP-9, LVEDVI and LVESVI at 12 months were significantly lower in the combination therapy group than in the trandolapril group. There were no significant differences between the valsartan group and combination therapy group, or between the valsartan group and the trandolapril group. Conclusions: LV remodeling might be suppressed in association with MMP-9 suppression in AMI patients treated with PCI and regular dose or half-dose-combination of RAS inhibitors. Furthermore, a half-dose-combination might suppress LV remodeling more effectively than trandolapril alone.  (Circ J 2010; 74: 1158 - 1164)