Uniformity of myocardial protection during retrograde blood cardioplegia is still a controversial area. We conducted a study on electron microscopic changes in the myocardium during mild hypothermic retrograde cardioplegia (31-32 degrees C) in 12 patients undergoing coronary artery bypass grafting. Biopsies for electron microscopy were taken from the right and left ventricular myocardium before and at the end of aortic cross-clamping and after 15 min reperfusion. The intercellular junctions, intracellular and extracellular oedema, mitochondria, capillaries, nuclei and myofibrils were analysed separately in each specimen, using a semiquantative method with scoring from 0 (unchanged) to 3 (severe changes), and the total scores were correlated with the severity of right and left coronary artery disease and with ischaemia time during aortic cross-clamping. Mild to moderate ultrastructural changes occurred in the myocardium during the cardiopolegia, most typically myofibrillar injury and oedema. These changes increased during aortic cross-clamping and reperfusion, especially in the right ventricle. The total ultrastructural score for the right ventricle correlated negatively with the severity of right coronary artery disease at the end of cross-clamping. No such correlation was found in the left ventricle. Apart from one case of perioperative myocardial infarction, the clinical outcome was unproblematic. Myocardial structure thus was by and large well preserved during mild hypothermic retrograde blood cardioplegia, with the right ventricle seemingly somewhat less protected than the left.