The present authors describe sleep problems, including sleep apnoea and excessive daytime sleepiness (EDS), in subjects with Prader-Willi syndrome (PWS). The present paper reports a questionnaire study regarding sleep and behaviour in a group of 29 subjects with PWS, compared with an age- and gender-matched control group. Those with PWS suffered from sleep problems more frequently than the control subjects. Problems included EDS, snoring and early waking. Sleep problems in PWS were not associated with body mass index or weight. Excessive daytime sleepiness was a distinctive feature of the group with PWS, and behavioural disturbance in PWS children and adolescents was associated with EDS. Excessive daytime sleepiness seems to be characteristic of PWS, and may be related to problems with the sleep-wake cycle and hypothalamic dysfunction.