Among the many deleterious effects of cardiopulmonary bypass[CPB], deterioration of plasma lipoprotein is a significant problem. Break-down of lipoprotein releases free fatty acids into the systemic circulation which, in turn, can cause microembolism. This study conducted on 30 patients undergoing CPB, was designed to elucidate the changes of plasma lipoproteins during and after CPB. Blood samples were taken at pre-CPB, on-CPB 30 minutes and 60 minutes, postoperative 1st, 3rd, 7th days. The levels of total cholesterol and triglyceride were measured and lipoprotein was subdivided into three kinds on electrophoresis. During CPB, the amounts of plasma total cholesterol and triglyceride decreased significantly and were recovered to pre-CPB levels after postoperative third day. There was no significant correlation between the duration and the severity of decrease in plasma total cholesterol and triglyceride levels. During CPB, the fraction of VLDL decreased and that of LDL increased. After cessation of CPB, the fraction of LDL decreased and that of HDL increased. After postoperative 1st day, the fraction of HDL decreased and that of LDL increased.