A small proportion of patients with deep vein thrombosis develop recurrent venous thromboembolic complications or bleeding during anticoagulant treatment. These complications may occur more frequently if these patients have concomitant cancer. This prospective follow-up study sought to determine whether in thrombosis patients those with cancer have a higher risk for recurrent venous thromboembolism or bleeding during anticoagulant treatment than those without cancer. Of the 842 included patients, 181 had known cancer at entry. The 12-month cumulative incidence of recurrent thromboembolism in cancer patients was 20.7% (95% CI, 15.6%-25.8%) versus 6.8% (95% CI, 3.9%- 9.7%) in patients without cancer, for a hazard ratio of 3.2 (95% CI, 1.9-5.4) The 12-month cumulative incidence of major bleeding was 12.4% (95% CI, 6.5%-18.2%) in patients with cancer and 4.9% (95% CI, 2.5%-7.4%) in patients without cancer, for a hazard ratio of 2.2 (95% CI, 1.2-4.1). Recurrence and bleeding were both related to cancer severity and occurred predominantly during the first month of anticoagulant therapy but could not be explained by sub- or overanticoagulation. Cancer patients with venous thrombosis are more likely to develop recurrent thromboembolic complications and major bleeding during anticoagulant treatment than those without malignancy. These risks correlate with the extent of cancer. Possibilities for improvement using the current paradigms of anticoagulation seem limited and new treatment strategies should be developed.