Prevalence of autistic disorder (AD) has increased dramatically in the last three decades. The comorbidity of the disorder with attention deficit hyperactivity disorder (ADHD) is well established, but current diagnostic criteria do not allow a dual diagnosis. In Greece, neither the prevalence of autism nor its comorbidity has been adequately examined.

The aim of the present study was to explore the prevalence of AD in schoolchildren in Crete and to assess its comorbidity with ADHD.

The initial sample was consisted of 7,661 children randomly selected. The study was conducted in two stages. Screening was based on school records. Only children with diagnosis of autism from the formal child psychiatric diagnostic centers were included in the sample. For each autistic child, the Teachers Report Form and the ADHD-IV scale were applied.

Out of 7,661 children, 25 (0.3%) had AD. The estimated prevalence was 0.5% for boys and 0.1% for girls (p< 0.0001). Ten children (40%) fulfilled DSM-IV diagnostic criteria for ADHD. Age and gender were found to be associated significantly with the presence of ADHD. The most prevalent co-occurrent subtype of ADHD was the combined. There was not significant evidence of an association between academic impairment of autistic children and the presence of ADHD.

The prevalence of AD in schoolchildren in Crete, the risk factors and its comorbidity with ADHD were comparable to those reported in other countries worldwide. Younger age and male gender seems to be associated with the presence of ADHD in autistic children.