The clinical diagnosis of deep venous thrombosis is difficult: "signs of thrombosis" described by Homan are not reliable. Edema in the subfascial compartment, livid discoloration of the skin, congested foot veins in the upright position and the search for potential superficial collateral veins provide a more accurate diagnosis. It must be realized, however, that in about one third of the patients there will be unavoidable errors which include hematomas in the muscle compartments, posttraumatic swelling, compression of the veins by tumors, aneurysms or cysts, acute forms of lymphedema, erysipelas, and insufficiency of muscle pump in paresis. Non-invasive tests (Doppler-ultrasound, plethysmographic techniques) increase diagnostic accuracy of 80-95% provided that the thrombosis affect iliac or femoral veins. In the leg region only phlebography and the test using labelled fibrinogen are sufficiently accurate. The diagnostic steps are described in detail. They depend in part on local facilities, severity of disease and the therapy planned (anticoagulation alone, fibrinolysis, thrombectomy). The better the left expectancy and the severe the symptoms, the more thorough must be the diagnostic measures, including phlebography with a view to possible removal of the thrombi by medical or surgical means.