This study reports the incidence of head injury and other risk factors and their association with severe violent offending in 242 juvenile detainees in the custody of the New South Wales Department of Juvenile Justice from January to March 2003. One third of the sample self-reported that they had experienced at least one head injury in the past. The most common causes of head injuries were fights (43.5%) followed by sport (29.4%) then misadventure/other (27.1%). More than half of those reporting a head injury reported cognitive or behavioural problems associated with head injury. A history of head injury was significantly associated with severe violent offending. Other risk factors associated with severe violent offending were harmful and hazardous alcohol use and coming from a culturally and linguistically diverse background. Protective factors against severe violent offending were Aboriginality and severe conduct disorder. The most parsimonious explanation for the relationship between head injury and violent offending is that head injuries increase disinhibition of aggressive impulses, especially in the presence of harmful/hazardous alcohol use, which raises the risk of severe violence within an offence pattern.