Patients with venous thrombosis of the proximal limb veins (femoral or popliteal vein) will be investigated with ultrasound agent in supine position. 5 ml of the contrast agent sulfur hexafluoride is given intravenously into a vein of the dorsal foot. The measurements are done in a defined area, where the thrombus is fully occluding in color Doppler investigation. The regions of interest will be the vessel walls and the centre of the thrombus in cross section view. The signal intensities are measured at baseline and peak (in decibel) in the centre and in the peripheral part of the vein. 20 patients with acute deep venous thrombosis will be included in this pilot study and investigated at baseline, after 3 weeks and 3 months.