A total of 27 post-menopausal women were treated with hormone replacement therapy over a period of 6 months for climacteric symptoms. Serum total cholesterol, high-density-lipoprotein (HDL) cholesterol, low-density-lipoprotein (LDL) cholesterol and triglyceride concentrations were determined before therapy commenced and during the third and sixth treatment cycles. One group (13 women) was treated with 2 mg oestradiol valerate plus 7.5 mg megestrol acetate (EV + MA). The other group (14 women) received 2 mg EV plus 0.25 mg norgestrel (Cyclabil). The serum total cholesterol concentration decreased in both groups, the fall being more marked in that treated with Cyclabil. The serum LDL-cholesterol and triglyceride concentrations also decreased in both groups. The serum HDL-cholesterol concentration fell in the Cyclabil group but did not alter in the women treated with EV + MA. Our results suggest that the cyclic addition of megestrol acetate, a 17-alpha-hydroxyprogesterone derivative, to oestrogen therapy does not affect the serum HDL-cholesterol concentration, whereas norgestrel, which is a 19-nortestosterone derivative, causes it to decrease.