Sleep apnea syndrome is a recognized manifestation of Arnold-Chiari malformation that almost invariably has been described in association with other neurological findings. We report a 39-year-old man who presented with severe daytime hypersomnolence. A detailed neurological examination was completely normal. Polysomnography revealed moderately severe mixed central and obstructive sleep apnea and hypopnea with persistence of central apnea and hypersomnolence despite nasal continuous positive airway pressure therapy. A history of severe headaches precipitated by sustained laughter prompted the consideration of an Arnold-Chiari malformation (ACM). A type I ACM with no evidence of syringomyelia or hydrocephalus was confirmed by MRI and successfully decompressed by suboccipital craniotomy and upper cervical laminectomies. Post-operatively the patient's headaches and hypersomnolence disappeared and a repeat polysomnography showed marked improvement in the sleep-disordered breathing. Physicians should be aware that severe sleep- disordered breathing may be the initial and sole presenting feature of Arnold-Chiari malformation.