Introduction: In persons with severe to moderate intellectual disability (SID), no valid and reliable diagnostic instrument is available to determine PTSD. In this study, a new instrument was evaluated: the Diagnostic Interview Trauma and Stressors -- Severe Intellectual Disability (DITS-SID). Methods: DITS-SID, Aberrant Behavior Checklist and Child and Adolescent Trauma Screener were administered to relatives and caregivers of 26 adults with SID. Results: Inter-rater reliability and convergent validity of the DITS-SID are good. There is a moderate to strong correlation between PTSD symptoms and challenging behaviors. PTSD prevalence is high at 13%-19%. Differences in results between relatives and caregivers were found. Conclusion: PTSD may be overlooked in individuals with SID, preventing them from receiving appropriate treatment. This study endorses the importance of using information from both relatives and caregivers when determining PTSD and provides a first step for a PTSD assessment. Follow-up research in a larger sample is needed to further validate the DITS-SID.