Postmenopausal women are believed to have a higher risk of coronary artery disease than premenopausal women. In this study, we prospectively determined changes in coronary risk factors that were attributable to natural menopause in 541 healthy, initially premenopausal women 42 to 50 years of age. After approximately 2 1/2 years, 69 women had spontaneously stopped menstruating for at least 12 months, and 32 women had stopped natural menstruation and received hormone-replacement therapy for a period of at least 12 months. An equal number of age-matched premenopausal women in the study group served as controls. In women who had a natural menopause and did not receive hormone-replacement therapy, serum levels of high-density lipoprotein (HDL) cholesterol declined as compared with those of premenopausal controls (-0.09 vs. 0.00 mmol per liter; P = 0.01), and levels of low-density lipoprotein (LDL) cholesterol increased (+0.31 vs. +0.14 mmol per liter; P = 0.04). In menopausal women who received hormone-replacement therapy, HDL and LDL cholesterol levels did not change, but the levels of triglycerides (+0.42 vs. -0.04 mmol per liter; P less than 0.001), apolipoprotein A-I (+0.18 vs. +0.03 g per liter; P less than 0.01), and apolipoprotein A-II (+0.05 vs. -0.03 g per liter; P less than 0.05) increased as compared with premenopausal controls. Natural menopause did not affect blood pressure, plasma glucose or insulin levels, body weight, the total number of kilojoules consumed in the diet, or the total number of kilojoules expended in physical activity. These results suggest that a natural menopause has an unfavorable effect on lipid metabolism, which may contribute to an increase in the risk of coronary disease. Hormone-replacement therapy may prevent some of these changes.