Depression has been reported to be frequent in narcolepsy and has been considered to be variously a reaction to chronic sleepiness or an endogenous expression of the pathophysiology of narcolepsy. Supporting the latter possibility are reports of similarities between the nocturnal rapid eye movement (REM) sleep of narcoleptics and inpatients with endogenous depression. In a comparison of 25 consecutive narcoleptics and 25 age-matched outpatient primary depressives, significant group differences were found in nocturnal EEG sleep measures of sleep continuity, sleep architecture, and REM sleep. Twenty per cent of the narcoleptic sample met Research Diagnostic Criteria (RDC) for a past history of major or chronic intermittent depression, but 60 per cent did not meet RDC criteria for any present or past psychiatric disorder. These findings mandate a cautious reevaluation of the nature of depressive symptoms in narcolepsy and leave open the question of whether there are common neurobiological control mechanisms in narcolepsy and depression.