In 31 patients with more than 2 migraine attacks per month the prophylactic effect of tolfenamic acid and placebo was compared in a double-blind crossover study. The patients were treated for 10 weeks with each preparation, with a 2-week break in treatment between the preparations. Tolfenamic acid had a significantly better effect in relation to the preferences of the patients, the number of attacks, the total duration of the attacks, the degree of severity, the number of episodes of vomiting, and work capacity. Mild symptoms and complaints occurred in both treatment periods, but they were qualitatively and quantitatively similar. Laboratory investigations of blood, liver function, and kidney function showed no changes whilst on treatment with either preparation. It is therefore concluded that tolfenamic acid can be used in the prophylactic treatment of migraine.