We investigated the prevalence of sleepwalking using a well defined population previously used for epidemiologic investigations: the Finnish Twin Cohort. The study population consisted of 11,220 subjects aged 33 to 60 years, and it included 1,045 monozygotic and 1,899 dizygotic twin pairs. Questions on the frequency of sleepwalking were asked separately for occurrence in childhood and adulthood. Childhood sleepwalking was significantly more frequent in women ("often" in 2.8% of women and 2.0% of men and "sometimes" in 6.9% of women and 5.7% of men). As adults, sleepwalking had occurred in 3.9% of men and in 3.1% of women, and it was reported "weekly" in 0.4% for both genders. There was no significant difference in frequency between monozygotic and dizygotic twin individuals, either in childhood or adulthood. For sleepwalking in childhood the probandwise concordance rate was 0.55 for monozygotic and 0.35 for dizygotic pairs, and for adults, 0.32 for monozygotic, and 0.06 for dizygotic pairs. Those who reported never having walked in their sleep in childhood did so as adults rarely (0.6%), both men and women. Those who reported walking in their sleep often or sometimes in childhood did so as adults for 24.6% of men and for 18.3% of women. Of adult men sleepwalkers 88.9% had a positive history of sleepwalking in childhood, and in women, 84.5%. The proportion of total phenotypic variance attributed to genetic influences was 66% in men and 57% in women in childhood sleepwalking, and 80% in men and 36% in women in adult sleepwalking. Our results show that there are substantial genetic effects in sleepwalking in both childhood and adulthood.