Background: Autistic adults with intellectual disabilities (ID) seem to be particularly vulnerable to potentially traumatic experiences and post-traumatic stress disorder (PTSD). Furthermore, this population may be at risk for a different set of traumatic experiences than the general population. However, knowledge is sparse concerning PTSD symptom manifestations in individuals with severe ID. Method: Exploration of PTSD symptom trajectories and manifestations in an adult, autistic man with severe ID. Results: Altered arousal/reactivity and problematic avoidance were the most easily observable symptoms. Avoidance seemed to become more generalised over time, and the impact of PTSD on behaviour, level of functioning, and quality of life was severe. Conclusions: Negligence and coercion in services for autistic adults with ID may involve a traumatic potential for these individuals. Increased awareness of this risk is needed in service providers and mental health professionals.