This chapter discusses the case of a 17-year-old girl who was admitted for the evaluation of excessive daytime sleepiness (EDS). It presents the clinical history, examination, follow-up, treatment, diagnosis, and the results of the procedures performed on the patient. She suffered developmental delay, and was later diagnosed with mild mental retardation. Nocturnal polysomnography (PSG) and a multiple sleep latency test (MSLT) were carried out. The diagnosis was childhood-onset narcolepsy with cataplexy. The parents declined CSF testing for hypocretin. The onset of narcolepsy may occur with any of the four cardinal symptoms (excessive sleepiness, sleep paralysis, hypnagogic hallucinations and cataplexy), the most frequent being EDS. Childhood onset of narcolepsy is uncommon, but has been reported. The usual age of onset is mid- to late teens up to mid-20s. Occasionally patients may become ill after the age of 40.