The level of hypocretin-1 (orexin A) is reported to be low in the cerebrospinal fluid (CSF) of subjects with narcolepsy. We measured the CSF level of hypocretin-1 of two young hypersomniacs. The first subject was a 9-year-old boy who exhibited cataplexy, and was demonstrated to be human leukocyte antigen-DR2 (HLA-DR2) positive. The second subject was a 16-year-old boy who exhibited no cataplexy and was HLA-DR2 negative. When excessive daytime sleepiness ensued, appetite and bodyweight were increased in case 1 but not in case 2. The level of CSF hypocretin-1 was undetectably low and within normal limits in case 1 and case 2, respectively. It appears to be important to measure the CSF level of hypocretin-1 in the diagnosis and treatment of narcolepsy.