Lower extremity venous thrombosis (DVT) is the most common vascular manifestation of Behcet's syndrome (BS). Currently, Doppler ultrasonography (USG) is the most commonly preferred imaging modality in the diagnosis and follow-up of patients with acute and chronic DVT. Magnetic resonance (MR) venography, a quick and a non-invasive imaging modality, is successfully used to detect DVT in various settings. We had been unaware of studies with MR venography in BS. The aim of this study is to compare the diagnostic value of true fast imaging with steady-state precession magnetic resonance (True-FISP MR) venography and Doppler USG in the assessment of chronic DVT among patients with BS. 28 BS patients with chronic lower extremity DVT were studied. Common femoral (CFV) and femoral vein (FV) on both right and left sides were examined for the presence of thrombosis, recanalisation, collaterals and reflux. There are findings of chronic DVT in all Doppler USG images of 28 patients (45 of 56 FV and 35 of 56 CFV), while MR venography detects chronic thrombotic changes in 26/28 (93%) patients (43 of 52 FV and 28 of 52 CFV). Collateral veins are detected in 19 patients (19/28) with MR venography, whereas they are present in only 7 (7/28) with USG (P=0.003). Furthermore, patients with severe post-thrombotic syndrome are more likely to have collateral formation on the MR compared to those without (12/14 vs 7/14; P=0.043). Among patients with BS, MR venography might be an alternative or additional method to detect chronic thrombosis in the lower extremities.