There is evidence that Asperger syndrome is associated with delusional beliefs. Cognitive theories of delusions in psychosis literature propose a central role for impaired theory of mind ability in the development of delusions. The present study investigates the phenomenology of delusional ideation in Asperger syndrome. Forty-six individuals with Asperger syndrome participated and were found to have relatively high levels of delusional ideation, primarily grandiose or persecutory. Factors associated with delusional belief were anxiety, social anxiety and self-consciousness, but not theory of mind ability or autobiographical memory. The findings indicate that delusional belief is a prominent feature in Asperger syndrome, but do not support a mentalization based account. A preliminary cognitive model of delusions in Asperger syndrome is proposed and the theoretical and clinical implications of the findings are discussed.