It has been evident since the first publication by Armand Trousseau (1865) that there is correlation between venous thromboembolism (VTE) and malignant tumors. This correlation has also been shown by recent studies. In patients with malignant tumors not only the incidence of VTE is higher but the course of VTE is more severe and relapses are more frequent. The procedures applied for the management of tumors (surgery, chemotherapy, radiotherapy, tamoxifen, central venous catheters) also increase the risk of VTE. It is generally accepted in Europe and in the U.S.A. (and also this constitutes the basis of the Hungarian guidelines) to group surgical patients by the risk of VTE by taking into account the characteristics of both the patients and the procedures. Recommendations also agree that for patients with medium or higher risk pharmacological prophylaxis is needed. Heparin or low molecular weight heparins in risk dependent doses and duration are administered most frequently. According to the recommendation of the American College of Chest Physicians (ACCP) patients with malignant tumors should receive thromboprophylaxis in accordance with their actual risk category and independent of the treatment they are receiving (surgery or chemotherapy). In a placebo controlled study with prolonged pharmacological thromboprophylaxis for patients with operated malignancies 60% VTE risk reduction could be reached and this risk reduction could also been observed after 3 months while the incidence of hemorrhagic complications was not significantly increased. Recent guidelines recommend 4 week long prophylaxis for patients with malignancies.