In 14 patients undergoing coronary surgery, repeated atrial biopsies were obtained before and at the end of ischemia, and at 20 and 60 min of reperfusion. In half of the patients reperfusion was initiated with an abrupt rise in blood temperature and pressure, in the other half with a gradual rise. The biopsies were prepared for transmission electron microscopy and analyzed by stereological technique. In all biopsies, myocytic injury, as revealed by mitochondrial changes and intracellular edema, occurred following ischemia (P = 0.0003 and 0.007, respectively). The intracellular edema regressed following 20 min of reperfusion (P = 0.008). The myocytic mitochondrial changes persisted during reperfusion towards the end of the observation period (P = 0.0001). Interstitial edema increased following ischemia (P = 0.007) and persisted following 60 min of reperfusion (P = 0.009). The capillary part was significantly reduced after 20 min of reperfusion (P = 0.003), probably reflecting interstitial edema. Most changes were reversible in nature, although foci of irreversible changes were shown. In patients with a gradual start of reperfusion there was a significant regression of interstitial edema (P = 0.005) at 60 min reperfusion compared to the patients with an abrupt start, where the same changes seemed to persist or even increase. The study demonstrates that "reperfusion injury" occurs in human myocardium. It can be discerned from "ischemic" injury, and it may be reduced by a gentle mode of reperfusion.