Background: Autism spectrum disorder (ASD) includes symptoms that vary in severity and frequency between children. Consequently, multiple psychometric assessment procedures are recommended to detect an ASD, including scales which asses the presence and nature of ASD-related behaviour. However, to date, few studies have examined the relative and specific contribution which such behaviourally oriented scales can make to the diagnosis of individual children with ASD. Method: Cluster analysis was used to explore the common characteristics of a group of 53 preschool and elementary school children with an ASD, based upon scores on tests of cognitive ability, adaptive behaviour and behavioural checklists designed to measure the presence of typical ASD. Results: Data confirmed the expected variability in intelligence test scores. In addition, measures of adaptive behaviour and data from a behaviourally based rating scale suggested that children with Asperger's Disorder and autism might be reclassified into subgroups according to the presence of particular ASD-specific behaviours. Conclusions: As well as confirming that children with ASD vary in their adaptive behaviour and cognitive levels, these data emphasise the limited contribution such assessment procedures make to an understanding of the child's day-to-day behaviour and functioning, thus arguing for the inclusion of behaviourally based rating scales to develop ideographic intervention plans.