A case of Kleine-Levin syndrome with true hypersomnia and a case of sub-wakefulness are described. In both patients lumbar cerebrospinal fluid homovanillic acid, 5-hydroxy-indoleacetic acid, 3-methoxy-4-hydroxyphenylethylene glycol levels have been assayed during episodes of hypersomnia and normal sleep-waking cycles. Besides an increased 5-hydroxytrypta-mine turnover, mainly an increased dopamine turnover has been detected in both kinds of hypersomnia, and this finding was more remarkable in the case with sub-wakefulness. The probable role of dopamine in abnormalities in the sleep-waking cycle is discussed on the basis of results in experimental animal hypersomnias.