Objectives. After conventional balloon angioplasty (PTCA) for acute myocardial infarction in 77 patients (77 lesions), we used myocardial fractional flow reserve (FFR(myo)) to assess the endpoint of percutaneous coronary intervention (PCI) and to determine whether adjunctive stenting was required. Of these, a total of 37 lesions with FFR > or = 0.94 after PTCA received no further treatment (FFR-PTCA group), while the remaining 40 lesions (FFR < 0.94) underwent adjunctive stenting (FFR-stent group). A further 78 patients (78 lesions) comprised the control group; these patients underwent direct stenting without FFR measurement (stent-only group). The restenosis rate at 14-day discharge (mean time to discharge) was 5.1% in the two groups treated with FFR guidance (FFR-PTCA and FFR-stent), but was 0% in the control group (p = ns). There were no significant differences in reocclusion rates between the FFR-guided patients (1.7%) and the controls (0%). There was no incidence of in-hospital mortality or reinfarction in any of the groups. The number of balloons used (mean, 1.3 0.6 balloons for FFR patients versus 1.8 0.5 balloons for control patients) and the total cost of hospitalization and treatment ($16,213 versus $19,730 in U.S. currency; 1,945,571 998,726 yen versus 2,367,656 538,444 yen in Japanese currency) were both higher in the control group. Long-term survival rates were comparable in the two groups. These findings indicate that FFR guidance for PCI of acute myocardial infarction is a useful, low-cost technique that results in similar clinical outcomes as primary stenting.