Two randomised, double-blind, parallel-group, placebo-controlled clinical trials were conducted to assess the efficacy of sumatriptan tablets, 50 mg and 100 mg, for treatment during the mild-pain phase of a menstrually associated migraine among patients who typically experienced moderate to severe migraine preceded by an identifiable phase of mild pain. Subjects (n = 403 in Study 1 and n = 349 in Study 2) treated one menstrually associated migraine on an outpatient basis. The results demonstrate that sumatriptan tablets, 50 mg or 100 mg, were significantly more effective than placebo at conferring pain-free response 1 h and 2 h post-dose; migraine-free response (i.e. no pain and no associated symptoms) 2 h post-dose; returning patients to normal functioning 2 h post-dose; and conferring sustained freedom from pain from 2 through 24 h post-dose. Although the studies were not designed or statistically powered to show differences between the sumatriptan doses, a trend for slightly higher efficacy was observed for the 100-mg dose compared with the 50-mg dose on many measures. Both doses of sumatriptan were well-tolerated. The only adverse events reported in more than 2% of subjects in a treatment group were nausea, paresthesia, dizziness and malaise/fatigue, all of which were reported at incidences comparable to or slightly higher than those with placebo. Considered in the context of other findings, these data suggest that—with menstrually associated migraine as with non-menstrual migraine—optimal therapeutic benefit of sumatriptan tablets may be realised when they are administered during the mild-pain phase of an attack rather than delaying treatment until headache is moderate or severe.