Autism spectrum disorder (ASD) affects 20–30% of adults with intellectual disability. This group are vulnerable to challenging behaviour and mental health problems.

To explore the extent to which ASD affects challenging behaviour among specialist mental health service users with intellectual disability.

To identify predictors of challenging behaviour among adults who have intellectual disability.

A cross-sectional study of 92 participants from a specialist mental health service for adults with intellectual disability in the UK. The presence/absence of ASD was confirmed using the Autism Diagnostic Observation Schedule. Challenging behaviour was assessed using the Developmental Behaviour Checklist (DBC).

Participants with ASD (N=48) had higher total DBC scores than those without ASD (N=44; mean=54.2 vs. 29.2). ASD, severity of intellectual disability, age, presence of psychiatric disorder and total number of needs were entered as independent variables into a linear regression. The model accounted for 51% of the variance and was statistically significant (F(5,91)=18.1, p<0.001). Presence of ASD and total number of needs were the only significant predictors of challenging behaviour. Presence of ASD had the highest standardised coefficient (β=0.56).

Participants with ASD had significantly higher levels of challenging behaviour than those without ASD. Challenging behaviour was also independently associated with total number of needs. Understanding which service users with intellectual disability have higher levels of challenging behaviour than others despite receiving psychiatric treatment, and the extent to which having ASD is a contributing factor, should inform the development of more effective services and lead to improved outcomes.