Iliocaval venous obstruction (ICVO) can lead to severe chronic venous insufficiency. The aim of this study was to identify the prevalence and clinical predictors of ICVO and the accuracy of duplex ultrasound (DUS) for diagnosis of >50% ICVO compared with computed tomography venography (CTV). Patients with lipodermatosclerosis and healed and active venous ulcer were prospectively evaluated with DUS and CTV. The loss of respiratory variation in the common femoral vein (CFV) and reversed flow in the superficial epigastric vein (SEV) were evaluated by DUS. Percentage of venous obstruction on CTV was measured. The prevalence and clinical predictors of >50% ICVO were analyzed. Accuracy of DUS in diagnosis of >50% ICVO was evaluated. A total of 106 patients (135 limbs) were enrolled. Mean age was 58.8 ±12.8 years, and 57 (54%) patients were female. Previous history of deep venous thrombosis (DVT) in the same leg was documented in 19 (14.1%) limbs. More than 50% ICVO was found in 38 (28.1%) limbs. Independent predictors of >50% ICVO included previous history of DVT in the same leg (P = .001) and left leg (P = .001). For diagnosis of >50% ICVO, sensitivity and specificity of the loss of respiratory variation in the CFV were 13.2% and 100%, respectively; sensitivity and specificity of the reversed flow in the SEV were 23.1% and 87.2%, respectively. Patients with severe chronic venous insufficiency often had >50% ICVO. Left leg or limb with previous history of DVT was a clinical predictor of >50% ICVO. If the loss of respiratory variation in the CFV or reversed flow in the SEV was detected by DUS, the patient usually had >50% ICVO.