High density lipoprotein (HDL) and total cholesterol (TC) levels have been measured and their ratio compared in four groups of subjects - those with vascular disease, controls, middle-aged "keep-fit" enthusiasts and young physical education students. Each group has also been subjected to analysis of risk factors known for atherosclerosis. The HDL level was significantly raised in the Athletic and Exercise groups even though the latter were more overweight and had the highest total cholesterol level. This would appear to indicate that exercise may exert its protective effect against atherosclerosis, at least in part, by elevating the HDL level above a certain critical level. The HDL/TC ratio was significantly lower in the Vascular group and we would advocate that any ratio less than 20% is highly suspicious of atherosclerosis. We suggest that both the HDL level and HDL/TC ratio should be viewed together as reflecting the risk of atherosclerosis.