Zonisamide is a new antiepileptic drug with multiple mechanisms of action and a favourable pharmacokinetic profile. Preliminary data suggest that zonisamide may be effective in migraine prophylaxis. We evaluated the efficacy and tolerability of zonisamide for migraine prophylaxis in refractory patients. We reviewed the charts of adult patients with International Headache Society-defined episodic migraine (EM) or with transformed migraine (TM) according to the Silberstein-Lipton criteria, who had been treated with zonisamide at our out-patient clinic for at least 60 days. Demographic data, zonisamide dosage and duration of treatment were collected and analysed. Headache frequency, attack duration, headache severity and headache-related disability before and after treatment initiation with zonisamide were compared. Thirty-three patients were included in the study (average age 43.9 +/- 8.4 years; 23 (70%) with TM and 10 (30%) with EM). The patients had failed an average of 6.2 migraine prophylactic drugs prior to zonisamide. The average zonisamide daily dose was 337.9 +/- 146.3 mg and the average duration of treatment was 186.4 +/- 174.0 days. The average number of days with headache per month was reduced in the entire study population from 20.7 +/- 9.5 before zonisamide treatment to 18.0 +/- 11.3 after its initiation (P = 0.06) [in TM from 24.7 +/- 7.3 to 21.0 +/- 10.7 (P = 0.06); in EM from 11.6 +/- 7.6 to 11.0 +/- 9.7 (P = NS)]. No significant changes in other headache parameters were found. Fourteen patients (42.4%) reported adverse events (AEs), the most common of which was fatigue. Most patients (12/14, 85.7%) rated AEs as mild or moderate. In this group of refractory migraine patients, zonisamide therapy did not result in a statistically significant beneficial effect on headache or on associated symptoms.