The data about risk for bleeding complications during anticoagulation in cancer patients with different oncology treatment are conflicting. To investigate the rate of bleeding in the course of oral anticoagulants, during treatment of malignant diseases, we conducted a retrospective study including 75 patients on stable anticoagulation prior to commencing their different oncology treatment. All patients were treated according to the consiliar decision, made based on the localization and pathohistological findings of the malignant disease. During their treatment the regular laboratory monitoring of INR was done. Every dose of oral anticoagulants, INR changes, as well as the size and localization of bleeding were recorded. During all the malignancy treatment 22 (30%) of patients were overanticoagulated. In 15 (20%) patients it was associated with bleeding, while 3 (4%) of them had to be transfused with fresh frozen plasma to stop the major bleeding. Most bleeding complications occurred in the group of patients treated with chemotherapy or with analgesics in the group with advanced disease. None of the bleeding complications were observed in patients treated with irradiation and surgery alone, where the bridging of oral anticoagulants with low molecular weight heparin was done before surgery. The oncology treatment of patients who take oral anticoagulants was connected with high risk for bleeding especially if chemotherapy as a therapeutic options was used. Therefore physicians should be aware of this risk and carefully monitor the intensity of anticoagulant therapy, especially during the first treatment weeks when the risk of bleeding is greatest.