Mesenchymal stromal cells (MSCs) have the potential to treat many myocardial diseases. We investigated whether these multipotent stem cells derived from bone marrow could be administered safely into the coronary circulation of healthy dogs. We injected about 0.5 million cells per kg bodyweight of early passage autologous MSCs into the left circumflex coronary artery of anaesthetised dogs. During administration, we noted ST segment elevation and T wave changes characteristic of acute myocardial ischaemia. 7 days later, macroscopic and microscopic evidence of myocardial infarction was noted. Histological sections of myocardium showed several scattered regions of dense fibroplasia accompanied by macrophage infiltrates only in areas where the MSCs were observed. We also noted raised plasma concentrations of cardiac troponin I and collagen fibril deposition in the lesions. These findings show acute myocardial ischaemia and subacute myocardial microinfarction after intracoronary arterial injection of MSCs into dogs.