Social skills were studied in 363 children with mild intellectual disabilities (ID) and 147 with moderate ID with and without autism (age 4 through 18). The objective was to investigate the value of the Children's Social Behaviour Questionnaire (CSBQ), as a measure of subtle social skills, added to a measure of basic social skills with the Vineland Adaptive Behaviour Scales (VABS), in identifying children with ID with or without autism. Children with mild and moderate ID, with and without autistic symptomatology were compared on basic social skills, measured with the Communication and Socialization domains of the VABS, and subtle social skills, measured with the CSBQ. Measuring basic social skills is not sufficient in differentiating between levels of ID. Communicative skills and subtle social skills, that concern overlooking activities or situations and fear of changes in the existing situation, seem to play a far greater role. Additionally, with respect to identifying autistic symptomatology, basic social skills do not contribute, as opposed to communicative skills and the tendency to withdraw from others. The results implicate that the CSBQ not only has specific value as a measure of subtle social skills to identify pervasive developmental disorders, but that the instrument also has a specific contribution to differentiating between the two levels of ID. Furthermore, our outcomes imply a slight difference between limitations in subtle social skills as mentioned by the AAMR (American Association on Mental Retardation 2002) and limitations in subtle social skills as seen in milder forms of pervasive developmental disorders. Clinical and theoretical implications will be discussed.