OBJECTIVE : To identify causes of recurrent hemifacial spasm (HFS) after initial microvascular decompression (MVD) and to assess the feasibility of redo MVD. METHODS : The study included 21 patients who underwent redo MVD over the last 2 decades. Their medical charts were retrospectively reviewed for preoperative medical history, previous and redo MVD intraoperative findings, and previous and redo MVD postoperative outcomes. RESULTS : Redo MVDs were the second operation in 20 patients and the third operation in 1 patient. The median interval between previous and redo MVD was 46.3 months (range, 14.4-188.2 months). Compression of offending vessels such as a vein or perforating artery located medial to or at the cisternal segment of the facial nerve was found to be a possible cause of previous MVD failure. MVD failure resulted from neglect of offending vessels in 10 patients, insufficient decompression in 7 patients, and untouched neurovascular compression sites in 4 patients. Spasm-free rates after redo MVD were 80.5% at 1 year and 90.5% in the last year of follow-up (median, 15.8 months; range, 3.6-152.0 months). Permanent hearing loss and facial palsy were each observed in 2 patients (9.5%). In addition, 1 patient each experienced cerebellar infarction and vocal cord palsy. CONCLUSIONS : Redo MVD remains a feasible treatment option for patients with HFS who failed to benefit from previous MVD, but it is associated with an increased risk of cranial nerve and vascular injuries.