To elucidate the etiology of the thrombogenic effects of high-dose medroxyprogesterone acetate (MPA) in the treatment of breast cancer, hematologic parameters were sequentially assessed in 12 patients receiving MPA 800 mg p.o. daily for 6 months as adjuvant hormone therapy after mastectomy. The results were as follows. (1) Coagulation system: levels of factor VII and fibrinogen decreased significantly, whereas factors II and IX increased significantly, with a shortened activated partial thromboplastin time. (2) Fibrinolytic system: plasminogen and alpha 2-plasmin inhibitor-plasmin complex increased, whereas fibrinogen degradation products remained low. (3) Anticoagulation system: antithrombin III increased significantly. (4) These changes were most marked after 2 or 4 weeks of MPA treatment, and returned to the pretreatment level one month after discontinuation of treatment. (5) No patients in this study developed thromboembolic disease during or after MPA administration. These results indicate that MPA may induce a hypercoagulable state, but this state does not directly lead to the development of thrombosis.