OBJECTIVE: To report the adverse effects associated with prolonged high-dose prednisone for the treatment of autoimmune inner ear disease (AIED). STUDY DESIGN: Prospective data collected as part of a multicenter, randomized, controlled trial for the treatment of corticosteroid-responsive AIED with methotrexate. SETTING: Tertiary referral centers. PATIENTS: One hundred sixteen patients with rapidly progressive, bilateral sensorineural hearing loss. INTERVENTION: All patients completed a 1-month course of prednisone (60 mg/d). In Phase 2, 67 patients with improvement in hearing underwent a monitored 18-week prednisone taper, resulting in 22 weeks of prednisone therapy at an average dose of 30 mg per day. Thirty-three patients were randomized to receive methotrexate in Phase 2. Thirty-four patients received prednisone and placebo. MAIN OUTCOME MEASURE: Adverse events (AE) in patients treated with prednisone only. RESULTS: Of 116 patients, 7 had to stop prednisone therapy during the 1-month challenge phase due to AE. Of 34 patients, 5 were unable to complete the full 22-week course of prednisone due to AE. The most common AE was hyperglycemia, which occurred in 17.6% of patients participating in Phase 2. Weight gain was also common, with a mean increase in body mass index of 1.6 kg/m2 (95% confidence interval, 0.77-2.3) during the 22-week steroid course. Patients entering Phase 2 were followed for a mean of 66 weeks. No fractures or osteonecrosis were reported. CONCLUSION: Although high-dose corticosteroids are associated with known serious side effects, prospective data in the literature are limited. The present study suggests that with appropriate patient selection, monitoring, and patient education, high-dose corticosteroids are a safe and effective treatment of AIED.