Clinical diagnosis of deep vein thrombosis (DVT) is unreliable, and treatment should not be undertaken without objective confirmation. The traditional 'gold standard' for the diagnosis of DVT has been venography, but ultrasonic imaging has now replaced venography as the new diagnostic standard in many hospitals. A variety of noninvasive physiologic tests are also useful in selected circumstances. These include plethysmography, Doppler flow studies, radioisotope tests, thermography and peripheral blood tests which reflect activation of coagulation or thrombolysis. We describe the clinical application of these diagnostic procedures, both for symptomatic patients suspected to have DVT and for asymptomatic patients at high risk for DVT. The selection of an appropriate diagnostic modality depends upon institutional capabilities, cost-effectiveness and individual patient circumstances.