In 31 patients with more than 2 migraine attacks per month, the prophylactic effect of tolfenamic acid [a prostaglandin inhibitor] and placebo was compared in a double-blind crossover study. The patients were treated for 10 wk with each preparation, with a 2-wk 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 vomiting episodes 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 while on treatment with either preparation. Apparently, tolfenamic acid can be used in the prophylactic treatment of migraine.