Challenging behaviour may not be part of the diagnostic criteria for Autistic Disorder but they are frequently exhibited by children and adults with this condition. Levels of challenging behaviours are highest in individuals with an autism spectrum disorder (ASD) and co-occurring intellectual disability (ID). The sample for this study consisted of 57 institutionalized adults with ID who either did or did not meet criteria for an ASD on a screening instrument ["Autism Spectrum Disorders-Diagnosis for Intellectually Disabled Adults" (ASD-DA)]. These two groups were compared on two parallel measures of challenging behaviour commonly used with this population: the "Behavior Problems Inventory-01" (BPI-01) and "Autism Spectrum Disorders-Behavior Problems for Intellectually Disabled Adults" (ASD-BPA). Consistent with previous research, individuals with ASD demonstrated higher levels of overall challenging behaviour and especially with regard to self-injurious and stereotypical behaviours. The convergent validity of these two scales was also demonstrated for the entire sample and by group.