© 2019 The Author(s) 2019. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved.Objectives: Lower extremity deep vein thrombosis (LEDVT) is a serious complication of Behçet's syndrome. Management constitutes mainly of administration of immunosuppressives, but the predictors of relapse and the optimal choice of immunosuppressives remain unclear. In this prospective study, we aimed to detect the risk and predictors of relapse and treatment response to different modalities. Methods: All Behçet's syndrome patients who presented with a first episode of acute LEDVT between 2010 and 2014 were prospectively followed with a standard protocol. Acute LEDVT was confirmed by Doppler ultrasonography. Serial planned Doppler ultrasonography assessments were performed during follow-up and additionally repeated in case of clinical suspicion. Recanalization rate was assessed at each visit. Our first-line treatment strategy consisted of AZA and CSs. IFN-alpha was used in patients who were refractory to or could not tolerate AZA or had concomitant eye involvement requiring further treatment. Results: Thirty-three patients with LEDVT (26 M/7 F) were prospectively followed for 40.7 ± 13.4 months. Among the 33 patients, 23 relapses were observed in 15 patients. Relapse rates were 29%, 37% and 45% at 6, 12 and 24 months, respectively. Among the possible predictors of relapse, poor recanalization was the only significant factor [hazard ratio 4.34 (95% CI 1.96, 10.0)]. Overall 29 patients were treated with AZA and 17 with IFN-alpha. The relapse rate was lower and recanalization rate was higher with IFN-alpha compared with AZA (12% vs 45% and 86% vs 45%). Conclusion: The relapse rate for LEDVT in Behçet's syndrome is high despite AZA treatment. IFN-alpha seems to be a promising agent for preventing LEDVT relapses and achieving good recanalization.