Oral sumatriptan in a dose of 100 mg aborts about 60% of migraine attacks within 2 h, but the headache may recur within 24 h. We investigated: (i) the incidence of headache recurrence after oral sumatriptan (ii) whether a second tablet of sumatriptan at 2 h increases initial efficacy and/or (iii) prevents headache recurrence and (iv) whether a further tablet of sumatriptan treats headache recurrence. In a randomized parallel-group clinical trial, 1246 patients treated one to three migraine attacks (with or without aura), with 100 mg oral sumatriptan. Two hours later they all took a double-blind randomized second table of sumatriptan (group I) or placebo (group II). Patients who initially improved, but then experienced headache recurrence took a further double-blind randomized tablet of sumatriptan or placebo. Proportions of patients who improved from moderate/severe headache to mild/none were similar in groups I and III at 2 h (55 vs 56%) and 4 h (80 vs 77%). Incidences of headache recurrence (moderate/severe-any grade of headache) and median times to headache recurrence were also similar: 22-32% at 16 h in group I and 25-33% at 16.5 h in group II. Sumatriptan was superior to placebo in treating headache recurrence: 74 vs 49% (p = 0.017) in group I and 70 vs 30% (p = 0.0001) in group II. Thus, one-fourth of patients experience headache recurrence at about 16 h after successful treatment of a migraine attack with 100 mg oral sumatriptan. A second tablet of sumatriptan at 2 h does not increase initial efficacy and neither prevents nor delays headache recurrence.(ABSTRACT TRUNCATED AT 250 WORDS)