Accurate diagnosis of deep vein thrombosis is important because untreated deep vein thrombosis can cause death or permanent impairment and because effective treatments are available. The approach to the diagnosis of deep vein thrombosis varies because of differences in local resources and expertise. Duplex ultrasonography with venous compression is the preferred initial test for the majority of outpatients who present with symptoms and signs that suggest acute deep vein thrombosis. Clinical outcome studies have shown the safety of withholding anticoagulants when two compression ultrasonography examinations are negative over a 5- to 7-day period. Alternative strategies, for example, combining clinical scores and D-dimer with compression ultrasonography, may also prove effective. In unusual circumstances, venography or even magnetic resonance imaging may be necessary.