Although elevated blood ketone body levels reduce major adverse cardiac and cerebrovascular events (MACCEs) risk in chronic heart failure, their relationship with acute myocardial infarction remains unknown. We investigated this relationship in patients with acute myocardial infarction. This single-institution retrospective observational study analyzed data from 114 patients with acute myocardial infarction at Nihon University Hospital from May 1, 2018, to November 1, 2022. The cut-off value of acetoacetate for the incidence of in-hospital MACCE was determined by drawing a receiver operating characteristic curve (ROC) and defining patients with acetoacetate above and below the optimal cut-off point value as ROC and low-acetoacetate (LA) groups, respectively. Propensity score matching was performed between the LA and high-acetoacetate (HA) groups. Sex, peak creatine kinase, lactate, and blood glucose were defined as confounding factors between in-hospital MACCEs and acetoacetate, and 1:1 propensity score matching between the LA and HA groups was used, resulting in 40 patients from both groups enrolled in the analysis. There was a significantly lower incidence of in-hospital MACCEs in the HA group (LA group: 9 [22%] vs HA group: 1 [3%], = 0.014). In conclusion, in acute myocardial infarction, elevated blood acetoacetate levels reduce the risk of MACCE.