The case of a 31-year-old female patient with severe unilateral headache, ipsilateral reddened eye, and rhinorrhea is reported. For years, the patient was treated according to the diagnosis of migraine headache. Because of the clinical picture typical of a cluster headache, the first probative oxygen application was carried out with rapid complete remission of the symptoms. This case illustrates how the targeted history and physical examination as well as the probational use of oxygen efficiently provided the rare initial diagnosis of cluster headache and facilitated effective treatment.