Myocardial infarction is a leading cause of morbidity and mortality worldwide. Recently, studies have been focused on myocardial regeneration and neovascularization through stem cell therapy. The aim of this study is to compare between the roles of human umbilical cord blood mononuclear cells (MNCs) and CD34+ stem cells in improving functional changes resulting from induced acute myocardial infarction (AMI) in rats. Methods: Forty albino Wistar rats divided into 5 groups were included; group 1 is control untreated group; group 2 received subcutaneous injection of 0.2 ml saline; group 3 is AMI group; group 4 is MNCs treated group and group 5 is CD34+ treated group. AMI was induced in groups 3, 4 and 5 by isoprenaline hydrochloride. One day after AMI induction cardiac enzymes were assessed to ensure occurrence of AMI. After one week groups 3 and 4 were treated by IV injection in the tail vein with a dose of 107 MNCs/rat and 106 CD34+ cells/rat respectively. After 4 weeks, rats were anesthetized by intraperitoneal injection of urethane in a dose of 1.25 mg/Kg (Iwamoto etal., 1987) and subjected to ECG monitoring and Echocardiography, then animals were sacrificed and hearts were removed for assessment of vascular endothelial growth factor (VEGF) mRNA expression. Results: One day after AMI induction, cardiac enzymes were significantly higher in groups 3, 4 and 5 compared to other groups verifying occurrence of AMI induction. ECG showed marked elevation of ST segment and presence of pathological Q wave in G3 compared to the control group. There was significant reduction in ST segment in rats treated with MNCs or CD34+ cells associated with the disappearance of pathological Q wave compared to rats with AMI. Ejection fraction (EF) was statistically lower in G3 when compared to G1, G4 and G5 indicating that the treatment with CD34+ cells and MNCs improve EF. There was no difference between the two treated groups in both ECG and EF indicating that the treatment with MNCs or CD34+ has the same therapeutic effect. VEGF mRNA gene expression in group 3 was slightly higher than control group and its expression increased significantly in both treated groups compared to control and AMI groups; indicating the formation of new vessels. However, there was no significant difference in both treated groups; indicating that both kinds of treatment have the same effect on the gene expression. Conclusion: Treatment with human umbilical cord blood MNCs and CD34+ cells can improve the chemical, electrical and functional alterations in the heart after AMI induction in a rat model. This improvement could be attributed to enhanced new vessel formation. Keywords: AMI, Human Umbilical cord blood, CD34+ cells, MNCs & VEGF.