We review the hypothesis that the incidence of coronary heart disease (CHD) is higher in men than in women due to differences in plasma lipoprotein risk factors between the sexes. Men and women appear to be equally susceptible to the effects of lipoprotein risk factors for CHD, and the difference between the sexes in lipoprotein risk factors for CHD appears to be consistent with their being, at least in part, responsible for the sex difference in CHD. This is apparent both when men and women of equal age are compared, and when age-related variations in the sex differences in plasma lipoproteins and CHD are considered. Differences between the sexes in lipoprotein concentrations are still present when sex differences in adiposity, cigarette smoking, physical activity, and diet are taken into account. Evidence relating these sex differences in CHD and lipoproteins to the effects of sex hormones is critically examined. It is commonly accepted that androgens induce changes in lipoprotein concentrations that would predispose towards CHD, whereas estrogens are held to have opposite effects. However, much of the evidence for this comes from studies of changes associated with administration of synthetic gonadal steroids or with changes in gonadal function. Studies of differences in lipoprotein metabolism in normal men and women are extremely limited. In males high-density lipoprotein (HDL) cholesterol levels fall at puberty, correlating with the rise in plasma testosterone concentrations. In females, HDL levels do not change at puberty, despite the rise in estrogen concentrations. Evidence for lipoprotein changes during the menopause, when estrogen levels decline, is equivocal. Similarly, the evidence for an increase in CHD incidence at the menopause is inconclusive. National mortality data indicate that the decreasing sex difference in CHD after 50 years of age is due to a declining rate of increase in men rather than to an acceleration in CHD incidence in women. In men the age-related increase in low-density lipoprotein (LDL) concentrations diminishes beyond 50 years of age, whereas in women the rate of increase remains unchanged. Studies of the effects of gonadectomy are of doubtful relevance in assessing the roles of sex hormones in CHD, and have not been performed with sufficient rigor to provide definitive conclusions.(ABSTRACT TRUNCATED AT 400 WORDS)