Multiple regression analysis of 16 risk factors, including serum apolipoproteins in angiographically measured coronary stenosis, was carried out in 239 consecutive patients (159 males and 80 females, ranging in age from 30-80 years and mean 56.4 years) who underwent coronary angiography for suspected coronary artery disease during the past five years (1981-1985). The risk factors (independent variables) were age, total cholesterol (TC), triglyceride (TG), high density lipoprotein-cholesterol (HDL-C), low density lipoprotein-cholesterol (LDL-C), HDL-C/TC, apolipoprotein (Apo) A-I, A-II, B, C-II, C-III, E, YRSMOK (average number of packs per day x years of smoking), weight index (WI), glucose tolerance (GT), and blood pressure (BP). Severity of coronary atherosclerosis was scored as the extent of disease seen at arteriography (coronary score: CS). The order of importance of risk factors to CS in the five groups of subjects studied were as follows. (1) All patients: YRSMOK>Apo A-I>TC>GT, (2) Male group: Apo A-I>TC>Age>GT, (3) Female group: TC, (4) Young group (age below 54 years): BP>YRSMOK, and (5) Old group (age over 55 years): YRSMOK>TG>TC>GT. The results clearly indicated the importance of Apo A-I but not other apolipoproteins including Apo B in males, and that of blood pressure in the young group of the patients studied.