This study investigates the relationship between nocturnal or morning headache and obstructive sleep apnea syndrome (sleep apnea). It is not known if headache of any type is more common in patients with sleep apnea than in other patients, but morning headache is a symptom of sleep apnea. A method is needed for identifying patients with chronic headache who might benefit from evaluation and treatment of sleep apnea. We performed a retrospective assessment of frequency of morning headache in patients grouped according to final diagnosis: sleep apnea (n=72), periodic leg movements of sleep (n=28), and psychophysiologic insomnia (n=42). Prospective overnight sleep studies were obtained in a different group of 19 patients who presented for evaluation of headache. We selected certain patient characteristics as possibly indicative of sleep apnea-related headache. The retrospective study showed that 24% of patients with sleep apnea had frequent morning headache, which was not different from the other groups. In the separate group of 19 patients with chronic headache and suspected sleep disorder, 17 had sleep apnea. Nasal continuous positive airway pressure was prescribed to 14 patients. Marked improvement in headache occurred and persisted in 4 patients and moderate improvement in 3. Responders to therapy were more likely to have vascular headaches than mixed or tension headaches, more severe sleep apnea, and a nocturnal or morning timing to their headaches. However, there was large overlap in severity of sleep apnea and likelihood of response. We conclude that morning headache is not more common in sleep apnea than in other sleep disorders. However, over 30% of patients with chronic headache and other symptoms of sleep apnea have significant improvement in headache after treatment of sleep apnea.