To assess the therapeutic efficacy of modafinil in the treatment of increased daytime sleepiness in patients with Parkinson's disease (PD). Double-blind, randomized, placebo-controlled crossover study with two 2-week treatment blocks, separated by a 2-week washout phase. Tertiary Parkinson's disease care center and sleep laboratory at university hospital neurology department. Fifteen patients with idiopathic PD and daytime sleepiness (Epworth sleepiness score (ESS) 10 or more). Administration of placebo or modafinil as a single morning dose in a randomized crossover order. The modafinil dose was 100 mg in the first, and 200 mg in the second treatment week. At baseline and at the end of each treatment block, sleepiness was evaluated using subjective (perceived sleepiness with the ESS) and objective measures (maintenance of wakefulness test). Twelve patients completed the study (9 male, 3 female; mean age 65.0 +/- 7.6 years, mean disease duration 6.8 +/- 4.1 years). Epworth scores were significantly improved with modafinil (3.42 +/- 3.90) compared to placebo (0.83 +/- 1.99; p = 0.011). Latency to sleep in the maintenance of wakefulness test was not significantly altered by modafinil treatment: 10.9 (3-40)/15.1 (2.5-40) minutes before/after placebo and 12 (2.6-40)/17.8 (4.2-40) minutes before/after modafinil (p = 0.139) [data given as mean +/- standard deviation or median (range)]. The results of this study suggest that modafinil improves daytime sleepiness in PD patients, at least on a subjective or behavioral level. Modafinil treatment may be considered for EDS in PD patients, in whom otherwise treatable causes of Excessive Daytime Sleepiness (EDS) are absent.