The efficacy of sumatriptan, a 5-HT 1 receptor agonist, in patients with migraine attacks occurring despite prophylactic treatment with oral dihydroergotamine, was assessed in a double-blind placebo-controlled study involving 76 patients. Thirty-seven patients were treated with a subcutaneous injection of 6 mg sumatriptan self-administered with an auto-injector and 39 with placebo given by the same route. Patients having inadequate relief were allowed to use a second injection of test medication I hour later and rescue treatment between 2 hours and 24 hours after the first dose. Headache relief was achieved within 2 hours after sumatriptan in 26 patients (70%) compared to 8 patients (21%) in the placebo group (P<0.0001). Of these patients, 19 (51%) and 3 (8%) were, respectively, pain free at this time. A second injection of sumatriptan was used by 8 (22%) patients compared to 30 (77%) patients in the placebo group (P<0.0001), whereas rescue medication was used respectively by 13 (35%) and 22 (58%) patients (P < 0.024). The adverse event profile of sumatriptan was not affected by the concomitant use of dihydroergotamine and side-effects were all minor and transient. Patient satisfaction was significantly higher in the sumatriptan group (75%) compared to patient satisfaction with placebo (16%). These results show that the high efficacy rate of subcutaneous sumatriptan and its safety profile remain unchanged in migraine patients receiving oral dihydroergotamine as pro-phylaxis.