A population-based case-control study of coronary heart disease (CHD) risk in young women attributable to cigarette smoking, hypertension, diabetes, and steroidal estrogen use was conducted among residents of Rochester, Minnesota. All newly diagnosed cases of CHD (sudden unexpected death [SUD], N = 18; myocardial infarction [MI], N = 90; and angina, N = 133) among female Rochester residents 40 to 59 years of age during the years 1960 through 1982 were identified, and two community control subjects were matched for age and duration of community medical record. The overall adjusted odds ratio (OR) for the association between steroidal estrogen use and definite CHD (MI and SUD) was 0.6 (95% confidence interval [CI] = 0.2 to 1.3). Smoking (OR = 5.1; 95% CI = 2.3 to 11.6), hypertension (OR = 4.8; 95% CI = 2.3 to 10.2), and diabetes (OR = 8.4; 95% CI = 1.6 to 44.5) were strong risk factors for CHD events. If considered causal, cigarette smoking accounted for 64% of all MIs and SUDs in the community, hypertension accounted for 45%, and diabetes accounted for 13%. Although steroidal estrogen exposure reduced CHD among these women by 14%, giving steroidal estrogens to all women in this age group might reduce the population rates of MI by as much as 45%.