OBJECTIVE: We aimed to identify clinical features in patients with severe headaches that predicted obstructive sleep apnea (OSA) and determine clinical and sleep study characteristics that predicted headache improvement with continuous positive airway pressure (CPAP). BACKGROUND: Many patients with headaches complain of sleep symptoms and have OSA. There is often improvement of headaches with CPAP treatment. METHODS: We conducted a retrospective chart review of all patients referred to adult neurology clinic for headaches and sent for polysomnography between January 2008 and December 2009. Follow-up ranged from 18 to 42 months. RESULTS: Eighty-two headache patients (70 females, 12 males) were studied. Mean age was 45±13 years (females 45±13, males 43±11) and mean body mass index was 32±9. Headache types included 17% chronic migraine without aura, 22% episodic migraine without aura, 32% migraine with aura, 21% tension-type headache, 6% chronic post-traumatic headache, 11% medication overuse headache, and 7% other types. All patients were receiving standard treatment for their headaches by their neurologist. Fifty-two patients (63%) had OSA. Increasing age, female gender, and chronic migraine without aura were predictive of OSA. Of the patients with OSA, 33 (63%) used CPAP and 27 (82%) were adherent to CPAP. Headache improvement was reported by 40 patients (49%) due to either standard medical therapy or CPAP. Patients with OSA who were CPAP adherent (21/27) were more likely to have improvement in headaches than patients intolerant of CPAP (2/6), those that did not try CPAP (8/19), and those who did not have OSA (16/30) (P=.045). Of the 33 patients who used CPAP, 13 reported improvement in headaches specifically due to CPAP therapy and 10 additional patients noted benefit in sleep symptoms. The presence of witnessed apneas (P=.045) and male gender (P=.021) predicted improvement in headaches due to CPAP. CONCLUSIONS: Headache patients should be evaluated for the presence of OSA. Treating OSA improves headaches in some patients.