The concept of idiopathic hypersomnia (IHS) was reviewed. The minimal criteria for IHS in the International Classification of Sleep Disorders (ICSD) are not clear enough to designate a clinical entity. We propose our criteria for IHS, which consist of 6 items. 1. Recurrent daytime sleep episodes with duration of longer than one hour that occur almost everyday for at least 6 months. 2. Absence of refreshed feeling after the daytime sleep episodes. 3. Length of nocturnal sleep is either normal or prolonged. 4. Total sleep time per day exceeds 10 hours. 5. Absence of cataplexy. 6. Does not meet criteria for other sleep disorders that account for the excessive daytime sleep episodes. Our clinical experiences with 16 Japanese IHS patients were described. Decrease in HLA Cw3 was found.