To investigate the focal myocytic and microvascular injury that develops during the first hour of reperfusion after hypothermic cardioplegic cardiac arrest, and to compare the influence of gentle versus more abrupt reperfusion, serial atrial biopsies were obtained from 14 patients undergoing uneventful coronary bypass surgery. The biopsies were taken before cardioplegia, at the start of reperfusion, and after 20 and 60 min of reperfusion. Transmission electron micrographs of biopsies examined by stereological techniques revealed endothelial injury. Following 20 min reperfusion there was accumulation of both red blood cells (p = 0.03) and polymorphonuclear leucocytes (p = 0.0004) were found. There was also intravascular accumulation of platelets (p = 0.008) and extravasation of red blood cells (p = 0.02), which increased throughout the observation period. If reperfusion was started with a gradual rise in temperature and pressure, the numbers of platelets in the microvessels were lower than following ordinary, abrupt reperfusion (p = 0.06). It is concluded that reperfusion injury is associated with microcirculatory disturbances with trapping of blood cells, changes which may be favourably modified by a gentle reperfusion technique.