All patients with proximal deep venous thrombosis are evaluated for study inclusion. The eligible patients are investigated in supine position in both legs with B-mode - compression sonography and Doppler spectral analysis. The vein segments are considered affected if they are incompressible and if no Doppler spectrum is assessable. The most proximal fully occluding thrombus area (in color mode) is defined and the localization is measured in cm from the groin or the popliteal line. The particular area of interest is investigated in cross section with color duplex sonography and power Doppler using the linear wideband scan head L7 (5-12 MHz) of the duplexsonography device GE LOGIC 9 (GE Healthcare Technologies, Ultrasound Milwaukee, WI, USA). One cineloop of 20 seconds each are saved in real time assessment. The sequences are saved in avi - and in dicom - format for each Doppler specification. The saved films will be evaluated offline by two different investigators using a visibility score.