The influence of tamoxifen on plasma lipids and lipoproteins was monitored in 46 postmenopausal and 8 premenopausal women treated for advanced breast cancer up till 6 months. Total cholesterol (total-C) did not significantly change. However, high density lipoprotein cholesterol (HDL-C) and the HDL-C/total-C ratio rose significantly. Low density lipoprotein cholesterol was significantly decreased. Triglycerides and free fatty acids did not change markedly. The concomitant rise of sex hormone binding globulin and thyroxine binding globulin indicates that the increase of HDL-C with prolonged use of tamoxifen is compatible with an intrinsic oestrogenic effect of tamoxifen on the liver. The increased HDL-C/total-C ratio lends no support to the concern that long-term administration of this anti-oestrogenic drug might lead to an increased cardiovascular risk.