Sumatriptan, a specific 5-hydroxytryptamine agonist, is a novel acute treatment for migraine. The efficacy and safety profiles of sumatriptan have previously been demonstrated in several controlled short-term studies. However, because migraine is a recurrent disorder which may persist throughout adult life, sustained efficacy and tolerability are essential if sumatriptan is to be of value in clinical practice. These aspects were therefore evaluated in a programme of three 12-month studies. Sustained efficacy with long-term use of single 100-mg oral doses has been demonstrated in an open study in which 288 patients treated 8,094 migraine attacks. The long-term safety profile of oral and subcutaneous sumatriptan has been evaluated in a total of 849 patients who treated 24,907 migraine attacks in studies lasting up to 1 year. Sumatriptan was well tolerated. Adverse events did not differ qualitatively or quantitatively from those in short-term studies, irrespective of the frequency of attacks or the number of doses used. Migraine attacks were effectively treated with doses less than the recommended maximum and there was no evidence of any adverse effect on attack frequency. In long-term studies the high efficacy of sumatriptan is maintained, and the adverse event profile is unchanged and unaffected by attack frequency.