We studied daytime sleepiness in 160 patients with Parkinson's disease and 40 normal subjects. We compared the prevalence of daytime sleepiness in patients who were taking levodopa alone, levodopa with bromocriptine, levodopa with ropinirole, and levodopa with pramipexole. We found that (1) all these anti-Parkinson drugs can cause daytime sleepiness; (2) 'dozing off' correlated highly with 'falling asleep without warning'; (3) after statistical adjustment for confounding variables there was no significant difference among the risks for any of these anti-Parkinson drugs causing daytime somnolence.