This multinational, multicenter, randomized, double-blind, placebo-controlled study in 169 patients investigated the effect of a 7-day period of preemptive treatment with oral sumatriptan (100 mg tid) on the frequency and severity of cluster headache attacks occurring during an established cluster headache period. Safety and tolerability were also assessed. Cluster headache patients who were not taking prophylactic medication and had experienced seven or more attacks in the preceding observation week, treated a cluster headache attack at home with subcutaneous sumatriptan 6 mg using an autoinjector device. Patients were then randomized to take sumatriptan 100 mg or placebo at 8-hourly intervals for a 7-day period. Cluster headaches occurring during this period could be treated 5 minutes after onset with rescue medication (100% oxygen or simple analgesics). Diary cards were used to record details of the cluster headache pattern during the observation and study treatment weeks. Preemptive oral treatment with sumatriptan 100 mg tid for 7 days did not produce a significant reduction in the number or severity of cluster headache attacks occurring during an established cluster headache period. Oral treatment with sumatriptan 100 mg tid over a 7-day period was not associated with an increased or altered adverse event profile from that previously reported.