In medical patients, risk factors of leg venous thrombosis are not well evaluated. Cancer is considered as an important one. The aim of this study was to evaluate the role of intrinsic thrombotic risk (tumor hypercoagulable state) and external thrombotic risk (associate factors). We have made a prospective analysis of thrombotic venous risk factors in two medical populations with leg venous thrombosis: patients with cancer and patients without cancer. Risk of thrombosis depends on the thrombogenic importance of the risk factor and its chronicity or not. We assessed cancer and thromboembolic disease at the time of diagnosis and during a median follow up of 125.2 days. We included 31 consecutive cases of cancer (21 men, 10 women, mean age 63.8 years), and 50 consecutive cases of non cancer patients (32 men, 18 women, mean age 65.5 years), these two populations were not different. The classic risk factors of venous thrombosis were not frequent in cancer patient. Analysis of thrombotic risk showed that 61% of cancer patient group had venous thrombosis without classic thrombotic risk, as compared to 32% in non cancer patient group, showing the direct role of cancer in thrombosis (p < 0.01). The cancer was often aggressive and metastatic adenocarcinoma of various origins. The effect of chemotherapy is not clear, only hormonotherapy seemed to be responsible in two cases. Cancer hypercoagulability, defined by clinical characteristics, is a real risk factor of venous thrombosis but of low frequency. Indeed, the incidence of venous thrombosis in oncologic unit is rare (0.4%). Finally, thromboembolic disease in cancer patients is not different than in no cancer patients, Trousseau's syndrome is unfrequent. Prognosis is poor (40% death with 44.5 days of median survival), and antithrombotic therapy complications are frequent (bleeding 16%, oral anticoagulants resistance 20%).