The high density lipoprotein (HDL) cholesterol concentrations of frozen specimen obtained in 1972-73 are reported from 93 men aged 40-49 years who later developed coronary heart disease (CHD), and for 186 controls. Mean HDL cholesterol of CHD-patients was 7.9% lower than that of controls matched for smoking habits and serum concentrations of triglycerides and total cholesterol (p 0.05 for 82 men who had myocardial infarction, n.s. difference for 11 with sudden coronary death), and 10.2% lower (p 0.05) than that of controls who were not matched for the parameters mentioned. The present prospective study confirms that HDL cholesterol is inversely associated with the risk of developing CHD in middle-aged men.