This double-blind, placebo-controlled, parallel-group, multicentre, multinational, phase-III trial was designed to assess the efficacy and safety of a single subcutaneous injection of placebo, 2 doses of alniditan (1.4 mg and 1.8 mg) and 6 mg of sumatriptan in subjects with acute migraine. A total of 114 investigators from 13 different countries screened 2021 subjects. In total 924 patients were treated with placebo (157), alniditan 1.4 mg (309), alniditan 1.8 mg (141) and sumatriptan 6 mg (317). The lower number of subjects in the alniditan 1.8 mg group is due to the termination of this trial arm after the incidence of a serious adverse event and a subsequent protocol amendment. The number of subjects who were pain free at 2 h (primary endpoint) was: 22 (14.1%) with placebo, 174 (56.3%) with alniditan 1.4 mg, 87 (61.7%) with alnditan 1.8 mg and 209 (65.9%) with sumatriptan 6 mg. Alniditan 1.4 mg was significantly better (P < 0.001) than placebo and sumatriptan was significantly better (P = 0.015) than alniditan 1.4 mg. The number of responders (reduction of headache severity from moderate or severe headache before treatment to mild or absent at 2 h), was 59 (37.8%) on placebo, 250 (80.9%) on alniditan 1.4 mg, 120 (85.1%) on alniditan 1.8 mg, and 276 (87.1%) on sumatriptan. Response was significantly higher (P < 0.001) with alniditan 1.4 mg than with placebo, and significantly lower (P = 0.036) with alniditan 1.4 mg than with sumatriptan. Recurrence rates were: 22 (37.3%) with placebo, 87 (34.8%) with alniditan 1.4 mg, 35 (29.2%) with alniditan 1.8 mg and 108 (39.1%) with sumatriptan. Adverse events occurred in 577/924 (62.4%) subjects, i.e. in 62/157 (39.5%) with placebo, 214/309 (69.3%) with alniditan 1.4 mg, 91/141 (64.5%) with alniditan 1.8 mg and 210/317 (66.2%) with sumatriptan 6 mg. Sumatriptan was significantly better than alniditan 1.4 mg for pain free at 2 h. The difference, however, was small and clinically not important. For alniditan, a dose-dependent adverse event relationship was seen. The safety profile of alniditan 1.4 mg was similar to that of sumatriptan.