Serum high-density lipoprotein cholesterol (HDL cholesterol), total cholesterol and triglyceride concentrations were determined in 158 post-menopausal women following long-term oral hormone replacement therapy. Oestradiol valerate (2 mg/day) was taken by 53 of the women and oestriol succinate (2 mg/day) by 42 others. The duration (means +/- SD) of the oestradiol valerate therapy was 6.4 (+/- 2.9) yr and of the oestriol succinate therapy 6.4 (+/- 2.3) yr. The remaining 63 women received oestradiol valerate (2 mg/day) combined sequentially with norgestrel (0.5 mg/day). The average duration of treatment with this combination was 3.3 (+/- 2.4) yr. The control group comprised 100 post-menopausal women who received no hormone therapy whatsoever. The HDL cholesterol levels in the women receiving oestradiol valerate were higher than those in the controls (P = 0.001) and in the women on oestradiol valerate plus norgestrel therapy (P less than 0.001). The HDL cholesterol levels in the oestriol succinate group did not differ significantly from those in the controls. The women receiving oestradiol valerate in combination with norgestrel had lower serum HDL cholesterol concentrations than the controls (P less than 0.05). Serum total cholesterol and triglyceride concentrations did not differ in either oestrogen group from those in the controls, but were lower in the oestradiol valerate-plus-norgestrel group than in the controls (P less than 0.001). There were no differences in serum total oestrogen, oestrone, oestradiol and oestriol levels between control subjects with normal HDL cholesterol concentrations and those with low concentrations.