We conducted an open study on the efficacy of 50 mg of sumatriptan as an acute treatment for migraine without aura. We recruited 200 consecutive patients, with an established history of migraine without aura, presenting at a headache center. The patients were instructed to take half a 100-mg sumatriptan tablet for their next migraine attack, and to record details of their headache in a diary. The primary outcome of the study was headache relief from one migraine attack. Attacks were moderately intense (46%), moderate to severe (7%), or severe (47%). Total or partial benefit at 2 hours from the 50-mg dose was reported by 140 of 200 patients (70%). Thirty-six patients received no benefit from half a tablet, and 24 did not take sumatriptan, preferring their habitual medication. Side effects were few, mild, and short lasting. We conclude that the 50-mg oral dose is generally effective for migraine without aura attacks of both moderate and severe intensity and recommend this dose for all such patients. If, however, sumatriptan is ineffective at that dose it can be increased to a maximum of 100 mg.