Background: Post-traumatic stress disorder (PTSD) is common and treatable. There is extensive research on people of average intelligence yet little on individuals with developmental disabilities. Methods: We report two people with intellectual disability (ID) who experienced PTSD. The relevance of their developmental difficulties, social and communication profiles, attentional skills, and causes of these, to their presentations is discussed. Results: Both individuals have fragile X syndrome and severe ID. One has Diagnostic and Statistical Manual - 4th Edition (DSM-IV) autistic disorder; the other DSM-IV attention deficit-hyperactivity disorder. They experienced developmental and psychological regressions, new challenging behaviours and exacerbations of existing ones coincident with emotional trauma. PTSD symptoms and phenomena were identifiable despite intellectual and communicatory impairments. Conclusion: Presentation of PTSD is influenced by degree and cause of ID, social circumstances, social and communicatory skills, nature and timing of traumatic experience and subsequent management. The paucity of literature suggests it is missed frequently in individuals with ID who risk having problems misattributed to other causes with potential for inappropriate interventions.