We assessed the efficacy of sodium valproate as a prophylactic agent in migraine headache. A prospective randomized study was conducted in adult patients who previously derived no significant benefit from most conventional prophylactic therapy for migraine. Twenty-seven patients with a diagnosis of migraine with aura or migraine without aura from a headache clinic received low dose sodium valproate for 3 months. Response to therapy was defined as 50% or greater reduction in the frequency of headache. Plasma drug level monitoring helped to identify four noncompliers who were excluded from the study. Seventeen (71%) patients observed improvement within 4-6 weeks of medication and remained well for 12 weeks. They were further followed up for 12-24 months. Two patients for side effects and 1 for nondrug-related problems were withdrawn from follow-up study. Twelve patients (60%) maintained their response for 12 months or longer. Clinical improvement (percentage reduction in the frequency of migraine attacks) correlated inversely with the plasma drug levels at 13-24 months and daily dose of valproate, among the responders, suggestive of a possible therapeutic window. In other words, patients who do not respond to low dose valproate are unlikely to benefit from further increase in dosage.