Background: Many people with learning disabilities (LD) show aggressive behaviour, but the extent of the problem and its associated factors and effects are unclear. Methods: A cross-sectional analysis was carried out using interview data from 3065 adults with LD on the Leicestershire LD Register. Physical aggression towards others was defined as carers reporting frequent (more than three times per week) and/or severe episodes. Individuals with and without aggression were compared using multiple logistic regression models for potential physical and psychological factors. Results: Carers reported that 443 (14%) of adults were physically aggressive towards others. Men (P=0.001), younger individuals (P is less than 0.001), people with more severe LD (P is less than 0.001) and those in institutional settings (P is less than 0.001) had a significantly higher prevalence of physical aggression. People with Down syndrome had a lower prevalence of physical aggression (P is less than 0.001). After adjustment, we found no relationship between aggression and the presence of epilepsy or autism. Among psychological factors, symptoms of frustration (P is less than 0.001) and mood swings (P is less than 0.001) were associated with higher levels of aggression. Failure to cope among carers was reported by 14% overall: 42% of people caring for adults with aggression said they were unable to cope compared with 10% of those caring for adults without aggression. Conclusions: Physical aggression towards other people presents a significant challenge to carers of adults with LD. Further research is needed to identify aetiological factors with a view to finding effective interventions to reduce, and improve management of, this behaviour.