Cardiomyopathy induced by doxorubicin (DOX) was recognized at an early stage and also several years after drug administration. Mesenchymal stem cells (MSCs) have many properties that make them suitable for preventive and/or regenerative therapies. In this study, we evaluated the effect of MSCs in the functional and the structural improvement of DOX-induced cardiomyopathy in rats. Ninety adult male albino rats were randomly divided into three equal groups of thirty rats each: Group I (control): rats received normal saline. Group II (DOX- group): rats received DOX. Group III (DOX-MSCs group): rats received DOX for 2 weeks then human umbilical cord blood mesenchymal stem cells (hUCB-MSCs). Rats in all groups were evaluated for: physical condition, electrocardiography (ECG), and hemodynamic parameters. Serum cardiac troponin I (cTnI), malondialdehyde (MDA), total antioxidant capacity (TAC), and DNA fragmentation on heart tissue isolated DNA were estimated for evaluation of the mechanism and the extent of the damage. Hearts were examined histopathologically for detection of MSCs homing, structural evaluation, with counting of the collagen fibers for evaluation of fibrosis. DOX-administered rats showed significant functional and structural deterioration. DOX-MSCs treated rats (group III) showed improved functional and structural criteria with restoration of all biochemical indicators of cardiac damage and reactive oxygen species (ROS) to normal, as well. In Conclusion, hUCB-MSCs significantly ameliorated the cardiotoxic manifestations as shown by biochemical, functional, and structural cardiac improvement. J. Cell. Biochem. 118: 3119-3129, 2017. © 2017 Wiley Periodicals, Inc.