The incidence of cardiovascular disease is lower in women than in men, but is raised in men with prostatic cancer treated by estrogens. The antiandrogenic gestagenic drug, cyproterone acetate, is sometimes used instead of estrogens in the treatment of prostatic cancer. The cardiovascular risk associated with such hormonal treatment has been debated. Changes in plasma lipoproteins are related to the risk of cardiovascular disease and can be brought about by hormonal treatment. Plasma lipoproteins were therefore investigated during cyproterone acetate therapy in 15 men with prostatic cancer. Cholesterol (C), triglyceride, and phospolipid (PL) concentrations were determined in the very low density, low density (LDL), and high density lipoprotein (HDL) fractions. During treatment, mean body weight decreased by 1.2 and 1.9 kg after 2 and 8 weeks, respectively. The mean C concentration was lowered by 16%. The HDL C was reduced by a mean of 25%, and the LDL C decreased by means of 15% and 10%. The part of plasma C carried in the HDL fraction decreased slightly from 19 to 17% (mean) and the mean HDL C/LDL c quotient was lowered from 0.27 to 0.22. The mean PL concentration in plasma was decreased due to a mean reduction of HDL-PL by 17% and LDL Pl by 10%. A reduction of the HDL/LDL quotient implicates an increased risk for development of atherosclerotic diseases. However, it seems unlikely that a modest reduction in the HDL/LDL quotient, as obtained by cyproterone acetate, should have any great influence on the incidence of cardiovascular disease in elderly men with prostatic carcinoma.