Background: The prevalence of autism spectrum disorder (ASD) symptomatology is comparatively high in Cornelia de Lange syndrome (CdLS). However, the profile and developmental trajectories of these ASD characteristics are potentially different to those observed in individuals with idiopathic ASD. In this study we examine the ASD profile in CdLS in comparison to a matched group of individuals with ASD. Method: The Autism Diagnostic Observation Schedule (ADOS) was administered to 20 individuals with CdLS (mean age = 11.34; range = 6-13 years) and 20 individuals with idiopathic ASD (mean age = 10.42; range = 8-11 years). Participants were matched according to adaptive behaviour and receptive language skills. Results: Sixty-five percent (N = 13) of individuals with CdLS met the cut-off score for autism on the total ADOS score. Further analysis at domain and item level indicated that individuals with CdLS showed significantly less repetitive behaviour, (specifically sensory interests); more eye contact, more gestures and less stereotyped speech than the ASD group. The CdLS group also showed higher levels of anxiety. Conclusions: The comparison between CdLS and idiopathic ASD indicates subtle group differences in the profile of ASD symptomatology that are not accounted for by degree of intellectual disability or receptive language skills. These differences may not be evident when relying solely upon clinical and domain level scores, but may be distinguishing features of the ASD presentations in the two disorders. The findings have implications for the conceptualisation and assessment of ASD in individuals with genetic syndromes.