Patients with Prader Willi syndrome (PWS) often complain of daytime hypersomnolence. Because of reported daytime sleepiness and high prevalence of morbid obesity, these patients have been considered at risk for sleep related disordered breathing, but polysomnographic studies have been limited. We evaluated sleep and breathing polysomnographically in 24 PWS patients including 15 adults and 9 children. All adult patients completed MSLT testing on the day following the nocturnal sleep study. Both adult and children groups showed little or no sleep apnea, but REM related oxygen desaturation was quite common, its severity significantly correlated with increased obesity. Sleep patterns in both groups showed abnormal REM sleep cycles with variable REM latency (at times significantly shortened) and fragmented REM sleep with multiple brief REM periods. REM sleep abnormalities were still present in some patients without REM related desaturation. As a group, patients with PWS demonstrated pathological somnolence as measured by MSLT, which correlated with nocturnal sleep efficiency but not with nocturnal REM latency. It is hypothesized that the abnormal sleep findings in PWS reflect an underlying hypothalamic dysfunction characteristic of this syndrome.