We investigated whether the lymphocyte-to-monocyte ratio (LMR) 48 hours after admission is related to 30-day and long-term mortality in patients with ST-elevation myocardial infarction (STEMI) who were treated with primary percutaneous coronary intervention (PCI). We evaluated 318 consecutive patients with STEMI who were undergoing primary PCI. The relationship between the LMR48h and all-cause mortality (30-day and long-term) was analyzed by categorizing the patients into tertiles (T) according to LMR48h—T1 (>2.46), T2 (1.67-2.46), and T3 (<1.67). The T3 group exhibited the highest risk of 30-day all-cause mortality (hazard ratio [HR]: 8.093 [1.006-65.074]; P = .049). For long-term mortality, a significantly higher mortality risk was observed in both T2 (HR: 2.005 [1.021-3.939]; P = .043) and T3 groups (HR: 2.374 [1.160-4.857]; P < .001) compared to the T1 group (reference group). In multivariate analysis, these associations remained unaltered even after adjusting for confounders. A low LMR at 48 hours after admission may be independently associated with both 30-day and long-term mortality in patients with STEMI who were treated with primary PCI. This marker may be used for identifying patients with STEMI at high risk.