A clinical investigation was undertaken of the haemorheological effects of short-term administration of synthetic sex hormones. In a randomised controlled investigative trial, groups of 20 women taking ethynyloestradiol, norethisterone, combined norgestrel and ethynyloestradiol or no therapy had their blood viscosity and its major determinants measured before and after 3 months of treatment. Oestrogens and progestogens, singly or in combination, were found to cause a rise in blood viscosity. Oestrogens did so by raising haematocrit and plasma fibrinogen, parameters that are similarly raised in other conditions such as pregnancy and following surgery when venous thromboembolism is common. The synthetic progestogen, on the other hand, raised the blood viscosity by increasing the haematocrit and decreasing erythrocyte deformability, parameters that are similarly altered in occlusive arterial disease. The combined preparation raised blood viscosity by altering all three parameters. These observations indicate the pathways whereby various synthetic oestrogens and progestogens in oral contraceptives or replacement therapy may be associated with different types of cardiovascular pathology.