Are we missing obsessive–compulsive disorder in children with severe intellectual disability?

Obsessive-compulsive disorder (OCD) is being missed in children with severe intellectual disability. Their compulsive behaviours are routinely attributed to autism spectrum disorder or dismissed as challenging behaviour, without systematic consideration of OCD. I argue that this misattribution is predictable, consequential and correctable. The key diagnostic tool is the ego-dystonic/ego-syntonic distinction: OCD rituals are distressing to the child and a change from their baseline, whereas autistic repetitive behaviours are typically comforting. When this distinction is applied, and OCD is treated with modified exposure and response prevention and selective serotonin reuptake inhibitors, outcomes can be transformative. A dedicated evidence base for this population is urgently needed.