It is well documented that children with ASD have brain hypoperfusion, that is, a lack of proper blood flow to certain parts of the brain. Many parents report improvements in cognition, language, and anxiety. The brain is not the only organ affected by HBOT, and many parents have reported improved intestinal health (better bowel movements, diminished bloating, increased appetite) and improved sleep. There is some thought that HBOT may help ameliorate mitochondrial issues.
Hyperbaric treatment for children with autism: a multicenter, randomized, double-blind, controlled trial - read the full study here.
Here you will find basic information and links to a variety of HBOT articles.
There is also a yahoo group for parents interested in learning more about this therapy for their children.
One of the venues in the UK specialising in HBOT is in Morecambe.
You can visit their website here.