Preliminary reports have shown that hyperbaric oxygen (HBO) interrupts cluster headache (CH) attacks. In the present study, 6 of 7 patients with episodic cluster headache who were treated with hyperbaric oxygen experienced an interruption of the attack. In 3 of 6 responders the florid period of the cluster headache was interrupted. The other 3 patients remained without pain attacks for a period lasting from 3 to 6 days. In 6 different patients, a placebo treatment had no effect. The present findings clearly indicate that hyperbaric oxygen has not only a symptomatic effect on a single attack of cluster headache, but it also could prevent the occurrence of subsequent attacks.