Elastic compression stockings are useful for preventing post-thrombotic syndrome after deep venous thrombosis (DVT). Less is known about their effects on thrombus recanalization and the optimal timing for starting compression. This study investigated whether compression applied early was more effective than when started 2 weeks after DVT. Seventy-three patients with DVT were randomly assigned to elastic compression hosiery starting either immediately after diagnosis or 2 weeks later. After 14 and 90 days the residual thrombus was measured by compression ultrasonography, and venous patency and any pathological reflux were recorded. There were significantly more recanalized venous segments in the group treated with early compression. Recanalization of popliteal DVT veins, expressed as the reduction of vein diameter, was also better in the early compression group than controls (day 14, 6.5 +/- 3 versus 5 +/- 2 mm, P = 0.035; day 90, 3.7 +/- 3 versus 2.1 +/- 1.7 mm; P = 0.014). On day 14 the mean score for popliteal patency was significantly better for the early compression patients (1.0 +/- 0.6 versus 1.5 +/- 0.5, P = 0.0015). In conclusion, elastic compression applied immediately at diagnosis of DVT was safe and effective on the surrogate end-points investigated in this study. Longer follow-up in larger series is needed to verify the patterns of recurrence of DVT and post-thrombotic syndrome.