Sleeptalking is usually benign but chronic cases in adults may relate to psychopathology. We hypothesize substantial genetic influences in the liability to sleeptalking and an association between sleeptalking and psychiatric disorders. In 1990 a questionnaire sent to the Finnish Twin Cohort yielded responses from 1298 monozygotic and 2419 dizygotic twin pairs aged 33-60 years. We used structural equation modelling to estimate genetic and environmental components of variance in the liability to sleeptalking. Register data on hospitalization and long-term antipsychotic medication were used to assess psychiatric comorbidity. The occurrence of childhood and adult sleeptalking was highly correlated. A gender difference was only seen in adults, with sleeptalking being more common in males than in females. The proportion of total phenotypic variance in liability to sleeptalking attributed to genetic influences in childhood sleeptalking was 54% (95% CI, 44-62%) in males and 51% (43-58%) in females, and for adults it was 37% (27-46%) among males and 48% (40-56%) among females. An association with psychiatric comorbidity was found only in adult sleeptalking, and it was highest in those with adult-onset sleeptalking (odds ratio, 3.77; 95% CI, 2.32-6.17). Sleeptalking is quite a persistent trait, also being common in adults. There are substantial genetic effects on sleeptalking both in childhood and as adults, which appear to be highly correlated. In adults psychiatric comorbidity is about twice as common in those with frequent sleeptalking, compared to those with infrequent or no sleeptalking, but most cases of sleeptalking are not associated with serious psychopathology.