It is known that deep venous thrombosis (DVT) of the ilio-femoro-popliteal axis is frequently associated with irreversible damage to valvular competence of the veins and consequently with varying degrees of chronic venous insufficiency. Because preservation of the valvular function of deep veins can play an important role in preventing the postphlebitic syndrome we analysed and compared the long-term functional outcome of two equally large cohorts of patients treated either surgically for restoration of venous patency and valvular function (24 patients) or medically with heparin, oral anticoagulants and compression stockings (25 patients). The study was also intended to examine the impact of duration and extent of DVT as predictive factors of late outcome. Follow-up time was 7.6 and 7.9 years respectively, operative mortality nil. Assessment of venous function was based on clinical observations as well as on measurement of haemodynamic parameters. Non-fatal pulmonary embolism after onset of treatment occurred in both cohorts with an equal frequency of 13%. Patients operated on for ilio-femoral DVT were with few exceptions totally independent of any form of adjunctive hosiery which was in sharp contrast to the conservatively managed group. If onset of DVT had occurred more than 3 days earlier and extended from the ilio-femoral axis to the popliteo-crural level, surgery usually failed and patients were no better off than in the comparable medical group. The same pattern of late outcome was found for all other clinical and haemodynamic parameters; i.e. clinical signs of venous hypertension, valvular competence as judged by sonography, patient's self-assessment and the expelled volume and refilling time measured by dynamic plethysmography after standardised leg work. The mean expelled volume was 1.1 +/- 0.5 ml/100 g/min. for the surgical group treated early for ilio-femoral DVT and 0.7 +/- 0.5 ml/100 g/min for the corresponding medical group (P = 0.05). Recovery or refilling time was 50 +/- 21 s for the surgical group and 28 +/- 26 s for the medical group (P = 0.03). Thus, the clinical and haemodynamic effect of surgical thrombectomy was significantly superior to conservative management in ilio-femoral thrombosis treated within 3 days. For extensive thrombosis treated early the advantage of surgical thrombectomy was also evident, but the difference between the two treatment groups was not significant. The advantage of surgery was however totally lost in patients operated on for extensive DVT of long duration (i.e. greater than 3 days).(ABSTRACT TRUNCATED AT 400 WORDS)