The authors examined the development over time of excessive daytime sleepiness (EDS) in patients with PD by evaluating EDS among 142 patients in 1993 and 4 years later. Eleven patients were diagnosed with EDS in 1993. In all of these patients, EDS persisted 4 years later. During follow-up, 30 new patients had EDS (6% new patients per year). In 1997, 29% of the patients with PD had EDS. The development of EDS correlated with more advanced disease and dementia.