Apparently so, according to PETA
[...] scientific studies have shown that many autistic kids improve dramatically when put on a diet free of dairy “products”. One study of 20 children found a major reduction in autistic behavior in kids who were put on a casein-free diet (casein is a component of cow’s milk). And another study done by researchers at the University of Rome showed a “marked improvement” in the behavior of autistic children who were taken off dairy products.
They are also running an ad campaign.
Now -- I don't believe this for a minute myself, but they claim scientific accuracy. Can we look at the evidence and see what it actually says?