We describe multiple sleep latency test (MSLT) results in 27 adult patients with idiopathic Parkinson's disease (PD). Pathological sleepiness (i.e. mean sleep latency </=5 min) was common (40 of 134 nap opportunities), and sleep-onset REM periods were also observed (13 of 134 nap opportunities). These findings bore little relationship to disease specific variables (e.g. level of disability, medication use), or sleep architecture measures (e.g. total sleep time, sleep stage percentage's). Our findings speak against a simple association of excessive sleepiness and the quality and quantity of prior night's sleep, but rather, argue for primary impairments of waking arousal and REM-sleep expression in a sizeable subpopulation of PD patients.