Background People with psychosis have an elevated risk of violence. Aims To examine whether violent behaviour in adults with psychosis can be accounted for by psychotic symptoms or physical aggression in childhood. Method We used data from a prospective longitudinal study of a complete birth cohort born in New Zealand. When cohort members were 26 years old, information was obtained on past-year psychiatric diagnosis of schizophreniform disorder and on violent behaviour. Childhood psychotic symptoms were measured at age 11 years using a diagnostic interview, and childhood physical aggression was assessed by teachers when cohort members were aged 7, 9 and 11 years. Results Participants with schizophreniform disorder were more likely to be violent than participants without, even after controlling for sociodemographic variables and concurrent substance dependence disorders. Childhood psychotic symptoms were a strong risk factor for violence in adults with schizophreniform disorder, as was childhood physical aggression, allthough to a lesser extent. Conclusions Violence by individuals with schizophreniform disorder could be prevented by monitoring early signs of psychotic symptoms and by controlling childhood physical aggression.