Cancer procoagulant (CP) is a cysteine proteinase that may be produced by malignant and foetal tissue. The possible role of CP in the pathogenesis of cancer-related thrombosis has been suggested recently. The purpose of the study was to evaluate coagulation prothrombotic markers and their relation to CP concentration in the blood of patients with gastrointestinal adenocarcinomas (GIAC). The study group consisted of 45 patients with confirmed diagnosis of adenocarcinoma (stomach, 18 patients; colon, 27 patients) and without evident metastatic disease. In 24 patients further observation showed metastases. The control group for CP was composed of 10 healthy subjects. Blood samples were drawn on the admission day, before any treatment. Among 45 patients with GIAC, deep venous thrombosis was observed in two (4.4%). In all patients the CP activity in the serum was found, and the mean CP activity shortened the coagulation time almost three times compared with the healthy control group. Also, the mean thrombin-antithrombin complex concentration was above the normal range. A significant elevation of the mean prothrombin fragment 1+2 plasma content in this group of patients was noticed. Despite these observations, CP remained within the normal range and did not correlate with thrombin-antithrombin complex or prothrombin fragment 1+2 plasma concentrations. A positive correlation was observed between serum CP and fibrinogen concentration, and a negative correlation between CP and free protein S plasma content (P = 0.04 and P = 0.025, respectively). A negative correlation between activated protein C resistance ratio and protein C activity in the plasma was confirmed. Protein C activity in the plasma showed a correlation with free protein S plasma content. Analysis of factors influencing the activated partial thromboplastin time revealed the presence of antiphospholipid antibodies in seven persons from the study group (in three cases of IgG and in four cases of IgM class). Our data suggest that CP is a minor risk factor for deep venous thrombosis in GIAC patients. To confirm this, however, the number of patients and controls should be larger. After 3 years of observation, the follow-up in 10 living GIAC patients showed nobody with thromboembolic disease.