Prospective cohort study aimed at simplifying the diagnostic approach to symptomatic patients with the clinical suspicion of deep vein thrombosis (DVT) of the lower extremities. All patients will receive a pre-test clinical probability (PTP) and the determination of D-dimer. Patients with low PTP and negative D-dimer will have the diagnosis ruled out. All other patients will undergo compression ultrasonography (CUS) of the proximal vein system. Patients with negative CUS and either low PTP or negative D-dimer will have the diagnosis ruled out, while those with high PTP and positive D-dimer will undergo extensive ultrasound investigation of the calf vein system. All patients in whom the diagnosis of DVT is ruled out will be followed-up prospectively up to three months for documenting the development of symptomatic thromboembolic events.