Limited data exist on the prevalence and prognosis of isolated posterior ST-segment elevation acute myocardial infarction (STEMI), revealed with a posterior chest lead. Furthermore, the utility of a synthesized-V_7–9 lead in the diagnosis of STEMI is unclear; therefore, we aimed to evaluate its usefulness. We enrolled 142 consecutive patients with STEMI with the culprit lesion on the left circumflex artery (STEMI-LCx) undergoing percutaneous coronary intervention (PCI) between January 2009 and December 2019. We retrospectively checked the ST-segment change of both standard 12-lead and synthesized-V_7–9 lead in all patients with STEMI-LCx. Based on electrocardiogram (ECG) findings, isolated posterior STEMI that was only revealed in synthesized-V_7–9 lead was classified as “STEMI-LCx-synV_7–9” and the remaining as “STEMI-LCx-12ECG.” The prevalence of STEMI-LCx-synV_7–9 in patients with STEMI-LCx was assessed. The incidence of all-cause death, cardiac death, and mechanical complications within 30 days, 3 months, and 1 year was also assessed according to each STEMI-LCx. STEMI-LCx-synV_7–9 and STEMI-LCx-12ECG occurred in 10 (7.0%) and 132 (93.0%) patients, respectively. No significant difference was found in patients’ characteristics between the two groups. The patients with STEMI-LCx-synV_7–9 had significantly higher incidences of cardiac death within 3 months and 1 year (30.0% vs. 6.1%, P = 0.031, 30.0% vs. 7.6%, P = 0.050, respectively) and mechanical complications in each follow-up period (20.0% vs. 1.5%, P = 0.025) than those with STEMI-LCx-12ECG. STEMI-LCx-synV_7–9 was observed in 7.0% of the patients with STEMI-LCx. Our findings suggest that the synthesized-V_7–9 lead helps diagnose isolated posterior STEMI and might improve prognosis in patients with STEMI-LCx.