Introduction Narcolepsy and idiopathic hypersomnia are known to show hypersomnia related to central nervous system origin with symptom of excessive daytime sleepiness. But little is known about the differences of clinical characteristics between narcolepsy without cataplexy and idiopathic hypersomnia. The aim of this study is to compare the clinical, polysomnographic and multiple sleep latency test characteristics of narcolepsy without cataplexy and idiopathic hypersomnia. Materials and methods Seventy-nine narcolepsy with cataplexy patients and Seventy-one idiopathic hypersomnia patients were recruited at Sleep Center of St. Vincent’s hospital. The demographic, clinical data, the multiple sleep latency test data and polysomnographic findings from the time of their diagnosis were reviewed. Results Results indicated that Epworth sleepiness scale score and nocturnal sleep disturbance were not significantly different between two groups. 25.0% of narcolepsy without cataplexy patients showed hypnagogic hallucinations positive, while 35.0% of idiopathic hypersomnia patients were hypnagogic hallucinations positive with no statistical differences. 33.3% of narcolepsy without cataplexy patients reported sleep paralysis, while 28.8% of idiopathic hypersomnia patients experienced sleep paralysis. Results showed no significant differences between two groups in onset age of hallucinations or onset age of sleep paralysis. Mean sleep latency was shorter in narcolepsy without cataplexy, and the number of SOREMPs were lower in idiopathic hypersomnia. The sleep efficiency was significantly greater in narcolepsy without cataplexy patients. The REM latency was shorter in narcolepsy without cataplexy patients. The percentage of REM sleep was significantly greater in narcolepsy without cataplexy patients. Conclusion The narcolepsy without cataplexy and idiopathic hypersomnia showed no significant differences in clinical characteristics although they have significant differences in objective findings such as mean sleep latency, REM latency, and percentage of REM sleep. Acknowledgements No conflicts of interest.