No research has been conducted on PTSD manifestation in children with severe or moderate intellectual disability (SID; IQ 20-49) and no PTSD measurement instrument was available. This pilot explores the psychometric properties of a new diagnostic instrument to investigate trauma and to classify PTSD in children with SID. The Diagnostic Interview Trauma and Stressors-Severe Intellectual Disabilities (DITS-SID), Aberrant Behavior Checklist and Child and Adolescent Trauma Screener 3-6 were administered to parents and residential caregivers of fifteen children with SID. The inter-rater reliability of the DITS-SID is good to excellent. There are indications of a good convergent validity. PTSD prevalence seems high at 27%-33%. Self-injurious behavior was one of the most commonly reported PTSD symptoms. There is evidence that the PTSD manifestation of children with SID does not differ from that of children without SID. PTSD is probably underdiagnosed in this target group. In understanding challenging behavior, it is important to consider an underlying PTSD in order to provide appropriate treatment. The DITS-SID is potentially suitable for diagnosing PTSD. Follow-up research in a larger sample is needed.