The aims of this study were to evaluate plasma lipid, apoprotein and Lp(a) levels in patients with severe coronary atherosclerosis undergoing aorto-coronary bypass surgery (BP) and to relate these parameters to the involvement of one or more vessels. Seventy-seven male patients and 77 cardiovascular disease-free controls, matched for sex, age and body weight were studied. Higher triglyceride and apo B levels with lower HDL-cholesterol and apo A-I levels were found in BP patients in comparison with the controls. Lp(a) levels were slightly, but not significantly, increased. Moreover BP patients presented a significantly higher prevalence of HDL-cholesterol levels below 35 mg dl-1 (49.3% vs 22.1%) and Lp(a) levels above 70 mg dl-1 (10.4% vs 1.3%) than the controls. When patients were divided according to the number of coronary vessels involved (one, two or three), no significant difference was found, with a trend to increase in Lp(a) mean levels and in prevalence of Lp(a) levels above 30 and 70 mg dl-1 in more severely diseased patients. These results suggest that patients with severe coronary artery disease undergoing aorto-coronary bypass surgery show low HDL-cholesterol levels with high triglyceride levels. Moreover Lp(a) levels above 70 mg dl-1 are highly associated with severe coronary vessel stenosis.