Objective: We sought to determine if patients with Parkinson's disease (PD), taking dopamine agonists (DAs) and reporting unintended sleep episodes (SEs), exhibit physiologically defined daytime sleepiness and can thus be differentiated from those taking DAs but not reporting SEs. Methods: Twenty-four patients with abnormal Epworth Sleepiness Scale scores of > 10 who were taking DAs were enrolled into one of two groups: those with SEs (SE+, n = 16) and those without (SE-, n = 8). Three consecutive days of testing included two nights of polysomnography followed by the Multiple Sleep Latency Test (MSLT). Results: Overall frequency of pathological sleepiness (MSLT < 5 min) was 42% (10/24). Mean levels of sleepiness, frequencies of pathological sleepiness, and naps with stage 2 or REM-sleep were similar between SE+ and SE- groups. Sleep tendency was similar in patients prescribed pergolide, ropinirole, and pramipexole combined with levodopa. Polysomnography testing revealed no significant differences between the groups in total sleep time, sleep efficiency, sleep architecture, or presence of restless legs syndrome or periodic leg movements. There was no relation between degree of nocturnal sleep disturbance and level of daytime sleepiness. Conclusions: The results of this study suggest SEs in PD patients occur upon a background of excessive daytime sleepiness and are unrelated to nocturnal sleep or use of a specific DA. © 2003 Published by Elsevier B.V.