Some physicians prescribe corticosteroids as adjunctive therapy for patients with migraine headaches to decrease the rate of rebound headache. The efficacy of this practice has not been tested. Our objective is to determine the efficacy of single-dose dexamethasone as adjunctive therapy for emergency medicine patients with migraine headache in preventing headache recurrence at 3 and 30 days posttreatment. From November 2004 to November 2005, we conducted a multicenter, double-blinded, placebo-controlled randomized clinical trial of adult patients who met the International Headache Society definition of migraine headache. After informed consent, patients were randomly assigned to one of two groups: receiving either placebo or 24 mg dexamethasone intravenously. To ensure generalizability, all other aspects of patient care were left to the discretion of the emergency physician. Clinical and demographic information was obtained; and patients were subsequently contacted at both 3 and 30 days to determine headache recurrence, current functional disability, and need for return to the ED. Our primary outcome measures were the recurrence of migraine headache at 3 and 30 days. We used Fisher exact to test for statistical significance. A total of 115 patients were enrolled, with 16 patients lost to follow-up at 3 days and 3 additional patients lost at 30 days. Baseline characteristics as well as adverse event profiles were equivalent in both study groups. At 3-day follow-up, 45% (95% confidence interval [CI] 31%-60%) of the placebo group had recurrence of their migraine compared with 35% (95% CI 24%-48%) in the dexamethasone group (P = .68). At 30-day follow-up, this relative reduction in migraine recurrence decreased to a 4% difference between the 2 groups (P = .68). Limitations include small sample size and significant proportion lost to follow-up. A single dose of dexamethasone as adjunctive therapy for migraine headache does not decrease the recurrence of migraines at 3 or 30 days.