Excessive daytime somnolence (EDS) and quality of sleep were studied in 25 parkinsonian patients at baseline, when they had not yet received any antiparkinsonian medication, and after 1 year of treatment with dopaminergic drugs. EDS was measured by the Epworth Sleepiness Scale (ESS) and sleep quality by the Pittsburgh Sleep Quality Index (PSQI). At baseline, the ESS score was not different from that of age-matched healthy controls. The mean ESS score increased significantly after 1 year of follow-up, being more than 10 in 12 patients. The mean PSQI also increased significantly after 1 year of treatment, but there were no differences in the number of "bad sleepers" at baseline and at follow-up. In conclusion, EDS seems to emerge during the course of the illness, at least in a proportion of PD patients, and could represent another clinical correlate of the interaction between the ongoing neurodegenerative process and the side effects of drugs.