1. The purpose of the study was to evaluate the degree of thrombus regression, development of valvular insufficiency, impaired calf muscle pump function and clinical symptoms after a period of acute deep venous thrombosis. 2. Seventy patients with acute deep venous thrombosis, diagnosed by duplex scanning or venography, received treatment with heparin and oral coumarin derivatives according to a standard protocol. All patients wore graduated compression stockings during the whole study period. Duplex scanning was performed at diagnosis and 1 and 3 months later to measure thrombus mass and reflux. The supine venous pump function test was used to assess calf muscle pump function. 3. Three months follow-up was completed in 60 patients. In total 218 (28%) out of 780 vein segments were initially thrombosed and 134 (17%) could not be traced. A statistically significant reduction of thrombus mass was recorded throughout the study period. Total resolution of thrombosis in all vein segments occurred in 25% of the patients within 1 month and in 40% in 3 months. There was no difference in regression between the various proximal vein segments. Distal segments showed more regression than proximal segments. Reflux occurred in 27% of the initially thrombosed veins and in 15% of the patent veins. Patients who showed total resolution after 1 month had a significantly higher calf muscle pump function than patients without total resolution (70%pf vs 61%pf, P < 0.05). Patients with reflux in two or more segments had a significantly lower calf muscle pump function than patients with reflux in less than two segments (58%pf vs 69%pf, P < 0.05). 4. Using duplex scanning and the supine venous pump function test 3 months after an acute deep venous thrombosis, overall haemodynamic abnormalities and local site of valve incompetence could readily be identified. Patients with haemodynamic abnormalities might be at risk to develop the post-thrombotic syndrome. There was no difference in thrombus regression between the various proximal vein segments. Reflux was significantly more often seen in initially thrombosed veins. Thirty-five percent of the patients developed an abnormal calf muscle pump function after 3 months. Patients with early resolution of thrombus had a higher calf muscle pump function after 3 months.