A study of a clinic-based sample of 25 individuals (12 females, 13 males; age at diagnosis 14.4 years, SD 7.4 years; age range 4 to 33 years) with Down syndrome (DS) and autism spectrum disorders, demonstrates that autism is by no means rare in DS. Results showed that there was a considerable delay in the diagnosis of autism as compared with children with autism who did not have DS. In 11 participants medical factors were identified that were likely to be of importance in contributing to the development of autism, and in four further participants there were factors of possible significance. Such factors include a history of autism or autism-related disorders in first- or second-degree relatives (n=5), infantile spasms (n=5), early hypothyroidism (n=3), evidence of brain injury after complicated heart surgery (n=2), or a combination of these factors. It is important that autism is recognised, identified, and fully assessed in individuals with DS in order for them to receive appropriate education and support.