The long-term effects of tamoxifen on alterations in haemostasis which could lead towards thrombosis were investigated in 149 women who were disease-free for at least 5 years following mastectomy for breast cancer. All participants were randomized to receive tamoxifen as a post-surgical adjuvant treatment (89 patients, treated group) or not (60 patients, controls) for at least 2 years. 5.62% of the cases treated with tamoxifen suffered a venous thrombosis, while no thromboembolism was reported in the control group. No significant differences were observed between groups in the global clotting times, fibrinogen, fibrinolytic factors, or in the concentration of the main natural anticoagulants, antithrombin III (AT-III), protein C(PC) and protein S (PS). However, when the treated group was sub-divided, current users (n = 18) of the drug (median treatment duration 72 months) had significantly lower AT-III (P < 0.05) and PC (P < 0.05) activities, together with higher levels of plasminogen activity (P < 0.01) and tissue plasminogen activator antigen (P < 0.01), compared with 71 ex-users (who mostly received treatment for 2 years) and controls. We conclude that long-term treatment with tamoxifen for 2 or more years tends to reduce both AT-III and PC, a situation possibly predisposing towards thrombosis. Monitoring haemostasis in tamoxifen-treated breast cancer patients is therefore advisable.