Female hormones are known in influence serum lipoproteins. In post-menopausal women oestrogens decrease the concentration of low-density lipoprotein (LDL) and increase that of high-density lipoprotein (HDL), while progestogens may have the opposite effect. The risk of coronary heart disease (CHD) should consequently be decreased by oestrogens and increased by progestogens. We report here the changes observed in serum total cholesterol, triglycerides, and lipoproteins in post-menopausal women during sequential oestrogen/progestogen treatment. Oestriol and 17 beta-oestradiol were given alone for the first 12 days, in combination with norethisterone acetate (1 mg/day) for the next 10 days, and then in reduced amounts for the last 6 days of the 28-day cycle. Three different doses of the oestrogens were investigated (high, medium and low). A total of 177 normal post-menopausal women volunteered for random allocation to treatment or placebo groups. Blood samples were taken every 3 mth during the progestogen phase of the cycle. Serum total cholesterol was found to be 10-13% lower over a 3-yr period on the high oestrogen dose and 5 and 3% lower on the medium and low doses, respectively. No significant changes were seen in serum triglycerides. Determination of lipoprotein fractions showed that the reduction in total cholesterol was due to reduced LDL-cholesterol, the HDL-cholesterol levels remaining virtually unchanged.