The effect of a 2-week course of hyperbaric oxygen on both the duration and frequency of cluster headache attacks was tested in four patients suffering from chronic cluster headache with no clear response to pharmacological treatments. Two patients (two courses in one case) dramatically improved while on hyperbaric oxygen treatment, this positive response remaining for 2 and 31 days posttreatment. Case 3 only improved in frequency, while the remaining patient showed no benefit. These findings suggest that daily hyperbaric oxygen treatment can be used as a transient preventive treatment for desperate cluster headache sufferers.