In a previous report we showed that oestriol succinate administered to postmenopausal women caused fewer changes in haemostatic function over a four-month period than ethinyloestradiol. Potential longer-term effects were studied in postmenopausal women treated for osteoporosis with oestriol succinate for up to 12 months. Over this period there was no significant change in concentration of plasma coagulation factors, an increase in plasminogen concentration and euglobulin lysis activity, and an inconsistent increase in platelet sensitivity to aggregation induced by ADP and collagen. The relative lack of effect of oestriol succinate on coagulation function is encouraging with regard to the future incidence of thromboembolic complications of therapy.