Background and Objectives: Due to the high mortality of ischemic heart disease, many of these patients can be life-saving treatments.There are conflicting information on the effects of insulin in patients with myocardial infarction. We aim to evaluate the effects of insulin on infarct size in myocardial infarction thorough evaluating troponin I enzyme and echocardiography.Methods: This randomized clinical trial enrolled 74 patients with ST segment elevation myocardial infarction referred to Vaseie hospital of Sabzevar in 2009. Patients were categorized into two groups by block randomization and were treated with high dose of GIK (25% glucose, 50IU of soluble insulin per liter, and 80mmol of potassium per liter at 1ml/kg/hour) (GIK group) or normal saline (control group) as adjunct to thrombolytic therapy. We analyzed Plasma concentrations of troponin I, at baseline, 16 and 24 hours after admission. Echocardiography was done at 72 hours after admission. Data were analyzed Variables were compared using independent T tests and repeated measure ANOVA. Results: cTnI peaked to 20.13±12.46U/L in GIK group and to 20.11±10.62 U/L in controls (p=0.44). Left ventricular ejection fraction was 39% vs.41% in GIK vs. control, p=0.34. There was no significant difference between groups in cardiac enzymes and ejection fraction.Conclusion: In patients with myocardial infarction treated with streptokinase, insulin offers no effect on infarct size.