Purpose Little is known about the associations between Mild Intellectual Disability (MID) and Borderline Intellectual Functioning (BIF) and aggressive behaviour in general Mental Health Care.Aim. To establish the association between aggressive Behaviour and MID/BIF, analysing patient characteristics and diagnoses.Method 1174 out of 1565 consecutive in-and outpatients were screened for MID/BIF with the Screener for Intelligence and Learning Disabilities (SCIL) in general mental health care in The Netherlands. During treatment, aggressive behaviour was assessed with the Staff Observation Aggression Scale-Revised (SOAS-R). Results Forty-one percent of participating patients were screened positive for MID/BIF. Patients with assumed MID/BIF showed significantly more aggression at the patient and sample level (Odds Ratio of 2.50 for aggression and 2.52 for engaging in outwardly directed physical aggression). Conclusions We found an increased risk for aggression and physical aggression in patients with assumed MID/BIF. We recommend screening for intellectual functioning at the start of treatment.