Obesity, short stature, hypotonia and excessive daytime sleepiness are characteristic features of the Prader-Willi syndrome. Excessive daytime sleepiness has been attributed to obstructive sleep apnoea (OSA). To investigate the role of anatomical factors in OSA in the Prader-Willi syndrome, clinical and ENT assessment, radiology of the upper airway and polysomnography including sleep oximetry were done in 14 subjects. Excessive daytime sleepiness was present in eight of 14 subjects as determined by a mean sleep latency to non-rapid eye movement stage I-II of < 5 min and/or self-rating sleepiness score > 9 (Epworth Sleepiness scale). Seven subjects were snorers or mouth breathers and dental abnormalities were present in 11. Sleep apnoea, as determined by a combined apnoea-hypopnoea index of more than 10 respiratory events per hour was present in 12 of 14 subjects. On clinical assessment, the nasopharynx, oropharynx and hypopharynx were small in one subject. No subject had redundant pharyngeal mucosa or an enlarged tongue. However, radiological studies performed in the awake supine posture showed a slight reduction in the cross-sectional area in nine subjects at the oropharyngeal level and in four subjects at the nasopharyngeal level as compared with normal control subjects. Sleep apnoea and minor radiological evidence of narrowing of the upper airway are common in the Prader-Willi syndrome, although clinical otolaryngological examination is often unremarkable. Excessive daytime sleepiness occurs in approximately 50% of all patients with Prader-Willi syndrome. Although obstructive sleep apnoea is one important factor related to sleepiness, an additional central disturbance of sleep mechanisms is present.