N-Acetyl Cysteine (NAC) is a compound that increases the levels of Glutathione, the body's main antioxidant. Glutathione is a compound in the blood that is part of a natural defense system (the antioxidant system). Anti-oxidants protect the body from damage caused by internal toxins called free radicals. It is possible that children with Autism tend to have lower levels of glutathione, an important compound in our bodies that helps combat the effects of toxic free radicals. We hope that by studying the antioxidant system in more detail, we will increase our understanding of the reasons why people develop Autism so that we can design better ways to treat individuals with this condition. This study is meant to test the safety tolerability of N-Acetyl Cysteine and its effectiveness in the treatment of behavioral difficulties in children with autism. It will also examine the possible benefit of this agent in improving the core deficits in autism such as social deficits.