The relationship of future clinical coronary heart-disease (C.H.D.) to the plasma-high-density-lipoprotein (H.D.L.)-cholesterol concentration has been examined in a 2-year case-control follow-up study of 6595 men aged 20-49 years living in the municipality of Tromsø, Norway. Measurements were also made of the cholesterol concentration in lower-density (i.e., density less than 1-603 g/ml) lipoproteins, plasma-triglycerides, systolic and diastolic blood-pressures, relative body-weight, and cigarette consumption. Discriminant-function analysis showed that coronary risk was inversely related to H.D.L.-cholesterol concentration and directly related to density less than 1-063 cholesterol. These relationships were independent of each other and of the other measured variables, which showed no significant differences between the cases and controls. H.D.L. cholesterol made a three-fold greater contribution to the prediction of future C.H.D. than did density less than 1-063 cholesterol in this cohort of young men. These findings support the proposal that a low H.D.L. concentration is a common antecedent of clinical C.H.D. and is important in accelerating the progression of coronary atherosclerosis.