BACKGROUND Relatively few epidemiologic studies have focused on narcolepsy, a disabling sleep disorder with a strong association with HLA-DQB1 *0602. METHODS We sought to estimate the prevalence of narcolepsy using multiple overlapping techniques to identify residents of King County, WA who were 18 years or older with physician-diagnosed narcolepsy. Patients were entered into a registry and recruited into an epidemiologic study entailing interview and buccal scrapings to determine HLA-DQB1 *0602 status. Missing values were imputed to allow prevalence to be estimated based on all 425 patients entered into the registry between 2001 and 2005, whether they were recruited into the epidemiologic study (n=279) or not (n=146). RESULTS As of July 01, 2001, estimated prevalence per 100,000 of physician-diagnosed narcolepsy with cataplexy was 21.8 (95% confidence interval (CI): 18.8-24.8), similar to prior studies. The median age of onset was 14 (interquartile range: 10-18). For narcolepsy with HLA-DQB1 *0602, prevalence was 15.3 (95% CI: 12.8-17.9). Estimated prevalence was higher in women than men and in African-Americans than other racial groups. CONCLUSIONS These differences could reflect problems in identification and recruitment or may provide etiologic clues about narcolepsy. This study illustrates the challenges in performing population-based studies of narcolepsy.