This research considers the prevalence of iron deficiency in children with autism and Asperger syndrome and examines whether this will influence guidelines and treatment. Retrospective analysis of the full blood count and, as far as available, serum ferritin measurements of 96 children (52 with autism and 44 with Asperger syndrome) was undertaken. Six of the autistic group were shown to have iron deficiency anaemia and, of the 23 autistic children who had serum ferritin measured, 12 were iron deficient. Only two of the Asperger group had iron deficiency anaemia and, of the 22 children who had their serum ferritin measured, only three were iron deficient. Iron deficiency, with or without anaemia, can impair cognition and affect and is associated with developmental slowing in infants and mood changes and poor concentration in children. This study showed a very high prevalence of iron deficiency in children with autism, which could potentially compromise further their communication and behavioural impairments.