The effect of prophylactic treatment with sodium valproate in cluster headache was investigated in an open pilot study. Fifteen cluster headache patients were studied of whom two suffered from chronic cluster headache and thirteen from the episodic type. The dose used was between 600 and 2000 mg per day in two divided doses. Eleven out of the fifteen patients (73.3%) reacted favorably to the treatment. Nine reported a complete disappearance of pain and two, a marked improvement. Sodium valproate appears to be an effective drug in the treatment of cluster headache. A double-blind controlled study is needed for further evaluation.