Background: Aggressive behaviour (AB) and non-suicidal self-injury (NSSI) are common in people with mild intellectual disability or borderline intellectual functioning, leading to adverse consequences for themselves and those around them. Method: We investigated the relationship between AB (both total and physical in particular) and NSSI and risk factors in 125 residents in a treatment clinic using incident reports and standard clinical measurements. Results: There was a weak correlation between AB and NSSI, as well as between impulsivity and total AB, and between coping and AB and NSSI. However, NSSI, impulsivity and coping skills did not predict AB. Conclusion: Results do not corroborate those of other studies in this area. In future studies impulsivity, coping, aggression and NSSI may be measured using other instruments, and differences between people with and without intellectual disability regarding these variables may be explored.