As part of a controlled trial of the use of tamoxifen for the treatment of mastalgia, some of the metabolic and haematological effects of this agent were measured. A panel of haemostatic variables including prothrombin time, kaolin cephalin clotting time, fibrinogen, euglobulin lysis time, factor VII, factor VIII, protein C and anti-thrombin III were determined. In addition, levels of sex hormone-binding globulin and both total and free oestradiol were estimated. No alteration in clotting function was found during the administration of tamoxifen, although hepatic function did alter during this period with an increase in concentration of sex hormone-binding globulin. There was a significant increase in total oestradiol and free oestradiol although the percentage of biologically available free oestradiol fell slightly during the course of tamoxifen treatment. There was a slight reduction in low-density lipoprotein cholesterol with an increase in HDL2, a subclass of high-density lipoprotein (HDL) cholesterol, consistent with an oestrogen-agonist effect. These data suggest that tamoxifen administration does not adversely influence haemostatic mechanisms or lipoprotein metabolism in the short term.