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Background: Although in patients with cancer the risk of venous thromboembolism (VTE) is increased, the incidence is too low to routinely give prophylactic treatment. Procoagulant microparticles (MPs), especially tissue factor (TF)-bearing MPs, contribute to the risk of VTE in cancer patients. In the present study, we assessed the MP-associated procoagulant activity using a functional assay, the fibrin generation test (FGT), to identify cancer patients prone to develop VTE. Methods: As an ongoing study, plasma was collected from cancer patients, mainly with stage III or IV pancreatic, gastro-intestinal, breast or lung cancer. The MPassociated procoagulant activity was determined via the FGT with the addition of an inhibitory antibody to factor VII. The prolongation of the clotting time in the presence of anti-factor VII is a measure for the contribution of TF-bearing MPs to the clotting time. Patients were followed up for 6 months. Results: 100 patients were included, of which 77 have completed follow-up until now. The first 43 patients were used to establish a cut-off value of the FGT. Receiver operating characteristics showed that a prolongation of the clotting time of 13% after addition of anti-factor VII, was the optimal cut-off value. In the entire group, 8 of 77 patients (10%) developed VTE, of which 7 could have been predicted by the FGT. Using this cut-off value, 23 patients (30%) had a FGT-result above the cut-off (positive test) and 54 patients had a FGT-result below the cut-off (negative test). The prevalence of VTE was 30% in the FGT-positive patients and 2% in the FGTnegative patients (sensitivity 88%, specificity 77%). Conclusions: The FGT seems an excellent predictor for VTE in cancer patients. The next step will be to test the efficacy of prophylactic anticoagulants in patients with cancer and a high thrombosis risk based on the FGT