There is compelling evidence that led to the hypothesis of a role of BDNF in the development of autism including increased serum concentrations of BDNF in children with autism and identification of different forms of BDNF in families of autistic individuals. [...]
In this study the regulation of BDNF in the cerebellum of six autistic patients and six controls was studied by measuring the protein level of BDNF in post mortem tissues. The level of BDNF was significantly decreased in the autistic group compared to controls. Reduced BDNF in the cerebellum may be an indicator of aberrant brain development and growth in autism. (via.)
Additionally, the same page that quote came from had this little interesting tidbit about serotonin and autism.