The mortality rate from coronary heart disease is much higher among men than women except in diabetes mellitus, which appears to reduce this sex difference. It is hypothesized that the female advantage is due, at least partly, to the more efficient insulin mediated glucose homeostasis in females, an advantage lost in the diabetic state. The authors studied 170 young adult men and women aged 20-24 years from a population-based survey in Beaver County, Pennsylvania, in 1981-1982, in an attempt to further elucidate the sex-specific relationships between fasting serum insulin concentrations and several risk factors. Women who used oral contraceptives and subjects whose fasting serum glucose exceeded 110 mg/dl were excluded. Insulin was related to body mass index in both sexes (r = 0.31; p less than 0.01 for men; r = 0.26, p less than 0.01 for women) and to systolic blood pressure (r = 0.27, p less than 0.01 for men; r = 0.36, p less than 0.001 for women). Insulin was related to diastolic blood pressure in men only (r = 0.31, p less than 0.05). Multivariate analysis revealed fasting serum insulin to be an independent predictor of systolic blood pressure in both sexes and of diastolic blood pressure in men only. Insulin was inversely related to high density lipoprotein cholesterol only among men and this relationship appeared to be largely independent of body mass index and triglycerides. Results indicate that insulin concentration is associated with an adverse coronary heart disease risk factor profile especially among men, consistent with their excess risk of cardiovascular disease.