The clinical diagnosis of deep venous thrombosis (DVT) is unreliable. Phlebography, an invasive method, has gained wide diffusion and is considered as gold standard, but it has several draw-backs such as elevated costs and x-ray exposure. For these reasons, other, non-invasive techniques for diagnosing DVT have been looked for. Among them, CW-Doppler, occlusion plethysmography and, more recently, colour-Duplex-sonography have gained most acceptance. While the first two methods are able to diagnose with sufficient sensitivity and specificity proximal DVT, they are unreliable for isolated calf vein thrombosis. The colour-Duplex-sonography, on the other hand, produces results similar to phlebography for proximal thrombosis and succeeds in detecting isolated calf vein thrombosis with sufficient accuracy. We propose therefore the following non-invasive proceeding when confronted with the question of DVT: The first investigation to be done is a (colour)-Duplex examination. If one lacks such an infrastructure, an investigation with CW-Doppler or occlusion plethysmography has to be performed. If the results are positive, the patient will be treated. Otherwise, the exam will be repeated after five to seven days for CW-Doppler and plethysmography. If now the result is positive, the patient will be treated, otherwise, as with a negative Duplex study, the suspicion of DVT will be dismissed.