Objectives: To assess the long-term efficacy and safety of modafinil in patients with excessive daytime sleepiness (EDS) associated with narcolepsy.Background: Modafinil has been shown to be effective and well tolerated for treating EDS associated with narcolepsy in two large-scale, well-controlled, 9-week clinical trials.Methods: Four hundred and seventy eight adult patients with a diagnosis of narcolepsy who had completed one of two 9-week, double-blind, placebo-controlled, multicenter, clinical trials of modafinil were enrolled in two 40-week, open-label, extension studies. A flexible-dose regimen (i.e. 200, 300, or 400 mg daily) was followed in one study. In the second study, patients received 200 mg/day for 1 week, followed by 400 mg/day for 1 week. Investigators then prescribed either 200- or 400-mg doses for the duration of the study. Efficacy was evaluated using Clinical Global Impression of Change (CGI-C) scores, the Epworth Sleepiness Scale (ESS), and the 36-item Medical Outcomes Study health survey (SF-36). Adverse events were recorded. Data from the two studies were combined.Results: The majority of patients ( approximately 75%) received 400 mg of modafinil daily. Disease severity improved in >80% of patients throughout the 40-week study. At weeks 2, 8, 24, and 40, disease severity was 'much improved' or 'very much improved' in 49, 58, 59, and 58% of patients, respectively. The mean (+/-SEM) ESS score improved significantly from 16.5+/-0.2 at open-label baseline to 12.4+/-0.2 at week 2 and remained at that level through week 40 (P<0.001). Quality of life scores at weeks 4, 8, 24, and 40 were significantly improved versus open-label baseline scores for six of the eight SF-36 domains (P<0.001). The most common treatment-related adverse events were headache (13%), nervousness (8%), and nausea (5%). Most adverse events were mild to moderate in nature. A total of 341 patients (71%) completed the studies. Forty-three patients (9.0%) discontinued treatment because of adverse events.Conclusions: Modafinil is effective for the long-term treatment of EDS associated with narcolepsy and significantly improves perceptions of general health. Modafinil is well tolerated, with no evidence of tolerance developing during 40 weeks of treatment.