The activity of cancer procoagulant (CP), prothrombin time (PT), activated partial thromboplastin time (APTT), the concentration of thrombin–antithrombin complexes (TAT) and the concentration of fibrinogen were analysed in blood of breast cancer patients scheduled for surgery. The serum level of CP activity was dependent on the stage of the disease. The CP activity was increased in 72% of patients with an early stage of cancer and in only 20% of patients with an advanced stage of the disease when compared to the baseline level for non-cancer controls. In all patients PT remained at normal levels (80–120%). There was no significant change in APTT (27–39 s) in early stage cancer patients. Only one patient with advanced cancer had APTT shortened to 23 s. Also one advanced stage patient had significantly elevated level of TAT (14.96 μg/l); in all other patients the concentration of TAT remained at normal levels (1–4.1 μg/l). Forty-four percent of early stage cancer patients and 22% of advanced cancer patients had an elevated level of fibrinogen (Fg) (>350 mg%). However, there was no correlation between the level of Fg and the CP activity (P>0.05). The data suggest that: (1) serum CP activity increases at the early stage of breast cancer and decreases down to the normal level in the advanced stage of the disease; (2) there is no evidence of blood clotting activation in the early stage breast cancer patients; and (3) CP does not facilitate the activation of coagulation in the breast cancer patients or the level of such activation is below the sensitivity of assays used in the experiment.