Sleep disturbances and daytime sleepiness are well-known phenomena in Parkinson's disease (PD). Fifteen previously untreated PD patients underwent clinical evaluation, subjective sleep evaluation and polysomnographic evaluation (PSG) before and after a treatment period of mean 8 +/- 3.1 months with dopaminergic drugs. Both mean Unified Parkinson's Disease Rating Scale (UPDRS) total score and mean subset III of the UPDRS were significantly improved with dopaminergic treatment. PSG revealed that administration of dopaminergic drugs resulted in significant increase in mean percentage of stages 1 and 2. The mean Epworth Sleepiness Scale (ESS) score was significantly increased and mean Multiple Sleep Latency Test (MSLT) score was significantly decreased after dopaminergic treatment indicating subjective and objective daytime sleepiness. The differences in MSLT scores were best explained by a higher dose of L-dopa, whereas other variables such as disease duration, treatment duration, Hoehn and Yahr stage, sleep efficiency index or dopamine agonists did not increase the significance. In contrast, any of the variables appeared to explain ESS score variability. This study demonstrates that daytime sleepiness is not present in untreated patients but emerges later during dopaminergic treatment. Total daily L-dopa dose is predictive of objective daytime sleepiness. Furthermore, subjective assessment of sleepiness may cause underestimation of the severity of daytime sleepiness.