The features of idiopathic hypersomnia are not well defined. We reviewed clinical and laboratory information on 42 subjects with idiopathic hypersomnia and obtained detailed follow-up evaluations on 28 of them. Only 29% of subjects had 'classic' idiopathic hypersomnia with non-imperative sleepiness, long unrefreshing naps, prolonged night-time sleep, difficult awakening with sleep drunkenness and prominent mood disturbances. Thirty-two percent had clinical features similar to narcolepsy, i.e. irresistible sleepiness, short and refreshing naps, few problems with awakening and good response to stimulants, without cataplexy or any indication of abnormal REM (rapid eye movement) sleep. The other 39% had intermediate clinical characteristics. We found no increase in the frequency of the human leucocyte antigens associated with narcolepsy. Overall, response to stimulants was good in three-quarters of the patients and spontaneous improvement of sleepiness occurred in one-quarter. Possible aetiologies identified in 10 patients included viral illness, head trauma and primary mood disorder. Idiopathic hypersomnia is a rare syndrome in which clinical heterogeneity suggests a variable or multifactoral pathogenesis. Only a minority of cases correspond to classical descriptions. Stimulants are often beneficial and spontaneous improvement appears to be more common than in narcolepsy.