It has recently been suggested that increased lipoprotein (a) levels are an independent risk factor for coronary heart disease. It has also been reported that tamoxifen can induce changes in blood lipid values. In this prospective study we investigated the long term effects tamoxifen on the lipid profile, focusing on lipoprotein (a) levels. Thirty-eight postmenopausal women with breast cancer treated with tamoxifen at a dose of 20 mg daily were studied. The mean age was 61 years. Serum lipoprotein (a) levels and lipid parameters (total cholesterol, high and low density lipoprotein cholesterol, triglycerides, apolipoprotein A-I and B) were measured after an overnight fast on the 1, 3, 6 and 9 month of tamoxifen treatment. A progressive fall in median lipoprotein(a) levels from 8 mg/dl to 3 mg/dl (p < 0.001) was observed. In addition, a significant decrease in the total and low density lipoprotein cholesterol and in apolipoprotein B, as well as an increase in apolipoprotein A-l were also noticed. It is concluded that our data on serum lipoprotein(a) levels agree with the beneficial anti-atherogenic effects of tamoxifen administration on lipoproteins.