Background:In the setting of elective percutaneous coronary intervention (PCI), intravascular ultrasound (IVUS)-guided PCI is associated with a reduction in the incidence of target vessel revascularization (TVR), but the impact of IVUS on long-term clinical outcome in the setting of emergency PCI for ST-segment elevation acute myocardial infarction (STEMI) is still unclear.Methods and Results:The subjects consisted of 3,028 STEMI patients who underwent primary PCI within 24 h of symptom onset in the CREDO-Kyoto acute myocardial infarction registry. Of these, 932 patients (31%) underwent IVUS-guided PCI. Compared with the angiography-guided PCI without IVUS, IVUS-guided PCI was associated with significantly lower incidences of TVR (primary outcome measure; 22% vs. 27%, log-rank P<0.001) and definite stent thrombosis (ST; 1.2% vs. 3.1%, log-rank P=0.003). The cumulative incidence of all-cause death was not significantly different between the 2 groups. After adjusting for confounders, however, there were no significant differences between the 2 groups in risk for TVR (adjusted HR, 1.14; 95% CI: 0.86–1.51, P=0.38) and definite ST (adjusted HR, 0.58; 95% CI: 0.19–1.72, P=0.33).Conclusions:IVUS-guided PCI was not associated with a lower risk for TVR or ST in STEMI patients undergoing primary PCI. (Circ J 2016; 80: 477–484)