Reduced adenosine deaminase (ADA) activity has been reported in sera of autistic children relative to controls. Additionally, the Asn allele of the ADA Asp8Asn polymorphism has been associated with reduced enzymatic activity. Therefore, we studied this polymorphism in autistic children and controls from two Italian populations. We observed a significantly elevated frequency of the low-activity Asn allele in the total sample of autistic cases relative to controls (P < 0.00001), and in both study populations (P < 0.001 and P < 0.025). We suggest that this putative genotype-dependent reduction in ADA activity may be a risk factor for the development of autism.