Children with Autism Spectrum Disorders (ASD) and children with Obsessive Compulsive Disorder (OCD) were compared on a range of repetitive behaviours. Parents reported similar levels of sameness behaviour and repetitive movements in the clinical groups, although children with OCD engaged in more repetitive behaviour focussed around routines and rituals. Children with OCD reported more compulsions and obsessions than children with ASD; both groups reported more compulsions and obsessions than a typically developing comparison group. Types of compulsions and obsessions tended to be less sophisticated in children with ASD than those with OCD. Sameness behaviour was more prevalent in younger children with OCD, but for children with ASD, age was not significantly related to sameness behaviour, repetitive movements, compulsions, or obsessions.