The Ullanlinna Narcolepsy Scale (UNS) is a simple questionnaire-based method used to measure the symptoms of the narcoleptic syndrome. The 11-item scale (range 0-44) assesses the two main features of the narcoleptic syndrome, the abnormal sleeping tendency and cataplexy. The UNS sum score reliably distinguishes patients with the narcoleptic syndrome from patients with sleep apnoea, multiple sclerosis, and epilepsy. The mean score in patients with the narcoleptic syndrome was 27.3 (95% confidence limits 24.4-33.1); the sleep apnoea group with a mean score of 9.6 (95% confidence limits 7.2-12.0) came closest to this. Validation data were also selected from a large survey of non-institutionalized adults in Finland including groups with insomnia, excessive daytime sleepiness, sleep deprivation, sciatica, alcohol abuse, and high scores on a depression scale and on a scale of neurovegetative symptoms. With the lowest UNS score in the narcoleptic syndrome group (= 14) as the cutpoint, the sensitivity is 100% and the specificity is 98.8% in the subjects studied. The accurate assessment of the symptoms of the narcoleptic syndrome in a format suitable for questionnaire studies is essential.