Background: This study investigated the validity of conceptualising elevated stress in parents of children who exhibit challenging behaviour within the framework of posttraumatic stress disorder (PTSD). It was hypothesised that parents of children with autism spectrum disorder (ASD), and parents of children with a rare disease would endorse greater PTSD symptomatology than parents of typically developing (TD) children, and that challenging child behaviours would positively predict PTSD symptomatology. Method: The Life Events Checklist for DSM-5, Developmental Behaviour Checklist (Parent) and PTSD Checklist for DSM-5 were administered to 395 parents. Results: Significantly more PTSD symptomatology was reported by parents of children with ASD and parents of children with a rare disease than parents of TD children, and challenging child behaviours positively predicted PTSD symptomatology in both groups. Conclusion: A PTSD framework may validly explain elevated stress among some parents of children with ASD and parents of children with a rare disease, and has important implications for support delivered to parents by healthcare providers.