Objective:We evaluated the long-term impact of iliofemoral thrombosis (I-F DVT ) on walking capacity, venous hemodynamic status, CEAP class, venous clinical severity, and quality of life, and determined the prevalence of venous claudication. Materials and Methods: All patients with prior I-F DVT , assessed at our institution since 1990, were called for follow-up. Those with walking impairment due to arterial disease (ABI Ͻ 1.0 postexercise) or unrelated causes and those thrombectomized or thrombolyzed were excluded; 39 patients (22-83 years, median 46 years) were included. Median follow-up was 5 years (range 1-23 years). Investigation included classification in CEAP and Venous Clinical Severity Scoring (VCSS) systems, air-plethysmography (outflow fraction [OF], venous filling index [VFI], residual volume fraction [RVF]) and venous duplex, treadmill (3.5 km/h, 10%) to determine initial (ICD) and absolute (ACD) claudication distances, and quality of life assessment (SF-36). Nonaffected limbs of patients with unilateral I-F DVT (37 of 39) comprised the control group. Data are presented as median and interquartile range. Results: A total of 81% of limbs with I-F DVT had superficial and deep reflux and 19% superficial reflux; reflux in control limbs was 29.7% (P Ͻ 0.001) and 27% (P Ͼ 0.2), respectively; 43.6% (17 of 39; 95% CI, 27-60%) of patients developed venous claudication ipsilateral to I-F DVT (ICD: 130 m, range 105-268 m), compelling 15.4% (6 of 39; 95% CI, 3.5-27%) to discontinue treadmill (ACD: 241 m, range 137-298 m). Limbs with prior I-F DVT had a lower OF (37%, range 32.2-43%; P Ͻ 0.001), abnormally higher VFI (3.8 mL/s, range 2.5-5.7 mL/s; P Ͻ 0.001), and RVF (45%, range 32.5-51.5%; P ϭ 0.006), and clinical impairment in CEAP and VCSS systems (P Ͻ 0.0001). Patients with I-F DVT had impaired physical functioning (P ϭ 0.02) and role (P ϭ 0.033), general health (P ϭ 0.001), social function (P ϭ 0.047), and mental health (P ϭ 0.043). Conclusions:A total of 43.6% of those with prior I-F DVT developed venous claudication compelling interruption of walking in 15.4%. Prior I-F DVT caused outflow impairment and a large residual venous volume and reflux, resulting in marked clinical and quality of life compromise. Standardized challenge enabled discrimination of those with clinically relevant impairment.