Lipoproteins and apoproteins were measured weekly in a group of 18 post-menopausal women treated with a cyclical hormone replacement regimen comprising 28 days on conjugated equine oestrogens (0.625 mg/day) with the addition of norgestrel (0.15 mg/day) for the last 12 days of the cycle. There were no significant changes in total triglyceride, very low-density-lipoprotein (VLDL) cholesterol or high-density-lipoprotein (HDL) cholesterol levels. Low-density lipoprotein (LDL) and total cholesterol levels fell, the differences being significant after two weeks. The LDL/HDL cholesterol ratio also fell significantly over 1 week of treatment. There were no significant changes in either HDL2 or HDL3 cholesterol. The HDL2/HDL3 cholesterol ratio did, however, alter significantly, increasing during the oestrogen-only phase. Apart from this ratio, none of the parameters measured showed any significant differences as between the oestrogen-only phases and the oestrogen/norgestrel phases. There were no significant changes from baseline values in the levels of apoproteins AI, AII or B. The apoprotein AI/AII ratio was significantly increased, the levels being higher during the oestrogen phase of the cycle. There was no significant change in the apoprotein B/AI ratio. The apoprotein B/LDL cholesterol ratio showed a statistically significant increase after 4 weeks. There was no evidence of any cyclical changes. We conclude that the results of this study are generally favourable with regard to coronary heart disease risk but that the change in the apoprotein B/LDL ratio merits further investigation.