Objective: To determine the synergistic effect of glycemia and leukocyte count by leukoglycemic index (LGI) on prognosis in patients with myocardial infarction with ST segment elevation (STEMI). Methods: A prospective longitudinal study of 233 patients diagnosed with STEMI who were admitted to a secondary hospital was performed. Clinical data and cardiac complications (CC) were collected during the hospital stay, including glycemia and leukogram at admission, calculating the LGI (glucose (mg / dL) × leukogram / 1000). Results: Patients with CC had higher LGI values (1864±1273 vs 1145±586; p = 0.00) as well as those with failed thrombolysis (1145±586 vs 1074±492; p = 0.00). The area under the ROC curve of the LGI to predict CC was higher than glycemia and leukogram by separate in both diabetic (n = 44) and non-diabetic patients (n = 189) (see Figure). Pathological ILG increased four times the likelihood of complications (OR = 3.9, 95% CI 2.1 to 7.2, p = 0.00) remaining an independent predictor after multivariate analysis. Conclusions: There is a synergistic effect between the inflammatory response and hyperglycemia leading to a worse in-hospital outcome in patients with STEMI and it can be evaluated by LGI.