Inborn errors of purine metabolism have been implicated as a cause for some cases of autism. This hypothesis is supported by the finding of decreased adenosine deaminase (ADA) activity in the sera of some children with autism and reports of an association of the A allele of the ADA G22A (Asp8Asn) polymorphism in individuals with autism of Italian-descent. We tested the ADA G22A polymorphism in 126 North American affected sib-pair families but found no aberrant allele distributions in cases versus controls. Instead, we found an increased transmission of the G allele from fathers to affected children. Our findings suggest that the ADA G22A polymorphism plays a minimal role in susceptibility to autism in North American families.