Background: Aggressive behaviours represent major obstacles to the integration into society of individuals with intellectual disability (ID) and pose significant management issues for carers. Methods: The present study assessed the prevalence and severity of five types of aggressive behaviours in 3165 adult men and women with ID receiving services from three rehabilitation agencies in Quebec by surveying their carers using the Modified Overt Aggressive Scale. Results: The 12-month prevalence of aggressive behaviour was 51.8%: 24% property damage, 37.6% verbal, 24.4% self-oriented and 24.4% physical aggression, and 9.8% sexually aggressive behaviour, most of which being mild in severity. Only 4.9% of individuals displayed aggressive behaviour leading to injury of the victim. Few gender differences were observed. Conclusions: The capacity to document and assess the types as well as the severity of aggressive behaviour is thus critical, not only to better understand the correlates of various types of behaviours but also to orient intervention programmes whether they be prevention, assessment, monitoring or management of aggressive behaviour.