Thus far, the association between residual vein occlusion and immediate compression therapy and postthrombotic syndrome is undetermined. Therefore, we investigated whether compression therapy immediately after diagnosis of deep vein thrombosis affects the occurrence of residual vein obstruction (RVO), and whether the presence of RVO is associated with postthrombotic syndrome and recurrent venous thromboembolism. In a prespecified substudy within the IDEAL (individualized duration of elastic compression therapy against long-term duration of therapy for prevention of postthrombotic syndrome) deep vein thrombosis (DVT) study, 592 adult patients from 10 academic and nonacademic centers across The Netherlands, with objectively confirmed proximal DVT of the leg, received no compression or acute compression within 24 hours of diagnosis of DVT with either multilayer bandaging or compression hosiery (pressure, 35 mm Hg). Presence of RVO and recurrent venous thromboembolism was confirmed with compression ultrasonography and incidence of postthrombotic syndrome as a Villalta score of at least 5 at 6 and 24 months. The average time from diagnosis until assessment of RVO was 5.3 (standard deviation, 1.9) months. A significantly lower percentage of patients who did receive compression therapy immediately after DVT had RVO (46.3% vs 66.7%; odds ratio, 0.46; 95% confidence interval, 0.27-0.80; P = .005). Postthrombotic syndrome was less prevalent in patients without RVO (46.0% vs 54.0%; odds ratio, 0.65; 95% confidence interval, 0.46-0.92; P = .013). Recurrent venous thrombosis showed no significant association with RVO. Immediate compression should therefore be offered to all patients with acute DVT of the leg, irrespective of severity of complaints. This study was registered at ClinicalTrials.gov (NCT01429714) and the Dutch Trial registry in November 2010 (NTR2597).