Tonabersat is a novel benzopyran derivative that blocks the cortical spreading depression proposed to be associated with migraine attacks. The ability of single oral doses of 15, 25, 40 and 80 mg of tonabersat to relieve the symptoms of moderate to severe migraine was evaluated in 859 migraineurs enrolled in two dose-ranging, double-blind, randomized, placebo-controlled, parallel-group trials, one international and the other North American. In the international study, significantly more patients given tonabersat than given placebo experienced relief of headache pain at 2 h (15 mg, 36.8%; 40 mg, 40.7%), the principal efficacy variable, and at 4 h (40 mg, 63.0%) and complete abolition of headache at 4 h (40 mg, 34.3%). None of the primary or secondary efficacy variables indicated significant differences between tonabersat and placebo in the North American study. Tonabersat was generally well tolerated, with dizziness and nausea the most common side-effects. Serious adverse events were uncommon, and no patient withdrew from either study because of adverse events. These results suggest a possible interplay between tonabersat pharmacokinetics (the relatively long time required to reach maximum plasma concentrations) and patient characteristics (previous triptan exposure) in the management of acute migraine attacks. Based on the pharmacokinetics and actions on cortical spreading depression, tonabersat may have potential value in migraine prophylaxis.