The prevalence of narcolepsy is usually presented at about 50/100,000. There are, however, marked differences of about 2,500-fold between the lowest and the highest reported prevalence. This discrepancy is at least partly explained by differences in the study populations and methods. There are, however, no earlier populationbased epidemiological studies with polygraphically confirmed diagnoses. We studied the occurrence of symptoms resembling the two main manifestations of narcolepsy, i.e. abnormal sleep tendency and emotion-associated muscular weakness, in an adult twin cohort (n = 16,179) with a questionnaire. A total of 3.2% met the minimal diagnostic criteria of the International Classification of Sleep Disorders for narcolepsy. Eleven questionnaire items assessing the main manifestations of narcolepsy formed a measure called the Ullanlinna Narcolepsy Scale (UNS), which has been validated. The UNS score was calculated for 11,354 subjects. Those (n = 75) having a UNS score equal or higher than the lowest value in a narcolepsy patient group were studied. Thirty-one of them (fulfilling also the minimal diagnostic criteria) were interviewed, and those suspected of having narcolepsy (n = 5) were evaluated in the sleep laboratory. In three subjects the narcolepsy diagnosis was verified (all dizygotic, nonfamilial and human leukocyte antigen DR2/DQB-0602 positive), giving a prevalence of 0.026% in the adult Finnish (Caucasian) population.