The efficacy and safety of sumatriptan, a selective 5-HT1 like receptor agonist, were studied in a randomized, double-blind, placebo-controlled, parallel-group, multicentre, multinational clinical trial of 235 patients suffering a moderate to severe migraine attack. Patients were randomized to treat a single migraine attack at home with 6 mg sumatriptan or placebo using an auto-injector. If the migraine had not improved at 1 h, patients had the option of taking a second identical injection. The primary measure of treatment efficacy was based on a comparison of the number of patients in the two treatment groups who had a reduction in headache severity from severe or moderate to mild or none at 1 and 2 h. At 1 h, 77% of patients treating with 6 mg sumatriptan compared to 26% treating with placebo (p < 0.001) had mild headache or none. At 2 h, the response rates for all patients had risen to 83 and 30%, respectively. Of those patients requiring a second dose at 1 h, improvement to mild or no headache at 2 h was achieved in 61 % of patients receiving sumatriptan compared to 15 % of those receiving placebo. Other migraine symptoms were more effectively treated by sumatriptan and patients were able to return to work or normal activities earlier. Migraine recurrence within 48 h was a feature of 46% of attacks treated by patients with either treatment. Adverse events were more frequent using sumatriptan but were minor and transient. It is concluded that 6 mg sumatriptan, self-administered using an auto-injector, is an effective and well-tolerated acute treatment for migraine, with an efficacy rate similar to that seen when sumatriptan is administered subcutaneously by a physician.