Narcolepsy is characterized by the excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations. The symptoms of narcolepsy can appear all at once or they can develop slowly over many years. The four most common symptoms are excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations. In some cases, excessive daytime sleepiness is the only symptom. An-8-years-old boy applied to our hospital due to excessive daytime sleepiness, sleep attacks which have occurred at last 3 months when he was eating, watching TV. Furthermore, at once he slept and fell down when he was biking. His first complaints started 3 months ago as excessive tiredness and falling asleep after play. The patient was suddenly collapsing, with knobs on falling asleep when he laughed or cried. He had bitten his tongue so as to stop laughing. After the evaluation of the medical history and physical examination, he was diagnosed to have narcolepsy. Imipramine treatment was started. Here we point out that people with narcolepsy often exhibit a sudden loss of muscle tone in response to emotional stimuli. For example, surprise might result in buckling of the knees and sudden collapse. This symptom may occur in 60 percent of people with narcolepsy.