We examined the acute and long-term effects of coronary artery bypass (CABG) surgery on serum lipid, lipoprotein and apolipoprotein levels. One series of 34 patients having CABG surgery was studied pre-operatively and for six weeks afterwards, and another 22 patients were investigated before and two years after CABG surgery. None of the patients studied received any lipid-lowering drug therapy or specific dietary advice. In both groups, pre-operative serum lipoprotein (a) (Lp(a)) and serum triglyceride concentrations were raised and serum high-density lipoprotein (HDL) cholesterol and apolipoprotein AI (apo AI) were low compared to healthy people. Acutely, there were profound decreases of 40-60% in the serum levels of cholesterol (p < 0.001), low-density lipoprotein cholesterol (p < 0.05), triglycerides (p < 0.01), Lp(a) (p < 0.05) and apolipoprotein B (apo B) (p < 0.05). There was a small decrease in serum apo A1 (p < 0.05), and serum HDL cholesterol showed no change. All these variables regained their pre-operative values within six weeks. Two years postoperatively, serum Lpa was 40% less than its pre-operative concentration (p < 0.001) and HDL cholesterol had increased (p < 0.001). Triglyceride levels decreased (p < 0.02) when beta-blockade was withdrawn. The long-term decrease in Lp(a) following surgery is unlikely to be due either to stopping beta-blockers or to life-style changes. Myocardial ischaemia relieved by the operation may have been partially responsible for its previously raised concentration.