Headache recurrence (HR) may occur within 24 hours in approximately 40% of migraine attacks initially treated successfully with 6 mg subcutaneous (SC) sumatriptan. This may be due to the short plasma half-life of sumatriptan. We studied whether an additional dose of 100 mg oral sumatriptan 4 hours after treatment of a migraine attack with 6 mg SC sumatriptan could prevent HR. Patients (n = 667) treated up to three migraine attacks in a randomized, double-blind, placebo-controlled, parallel-group, multicenter clinical trial. For each attack, they initially took open-label 6 mg SC sumatriptan by autoinjector. Four hours later all patients took either 100 mg oral sumatriptan or matched placebo. Patients could take an additional optional oral dose of 100 mg sumatriptan to treat HR. The primary efficacy end point was the number of successfully treated patients without HR within 24 hours after the initial SC injection for the first study attack. Two hundred twenty-five patients were not assessable for HR, mainly because of protocol violations. Of 442 assessable patients, 82/212 in the sumatriptan-treated group (39%) and 89/230 in the placebo-treated group (39%) reported HR in attack 1. Median times to recurrence were 15.6 hours after sumatriptan and 10.3 hours after placebo (p = 0.006). One hundred mg oral sumatriptan taken 4 hours after 6 mg SC sumatriptan does not prevent HR but significantly delays time to recurrence.