Patients with cancer have a high risk of pulmonary embolism and deep vein thrombosis (DVT). The approach to clinically suspected deep vein thrombosis starts with the evaluation of the clinical probability of disease followed by the measurement of the circulating levels of the D-dimer. The presence of the thrombus is objectively demonstrated by compression ultrasonography. In patients without cancer and clinically suspected DVT, a low clinical probability combined with a normal d-dimer result exclude the presence of DVT and compression ultrasonography can be withheld. In patients with cancer, signs and symptoms of the cancer disease may mimic those of DVT shifting the result of the clinical probability scores towards high probabilities. In addition, the presence of cancer is part of the Wells score which further increases the chance to end up with a higher overall score and thus higher probability. Eventually, patients with cancer are more often classified as at high clinical probability than patients without cancer.