Inclusion Criteria:

• Age 4 to 12 years.
• Diagnosis of autistic disorder, Asperger's disorder, or PDD NOS.
• If taking concomitant psychotropic medications, the medication must be at a constant dose for 60 days with no dose changes planned for the duration of the trial.
• Able to swallow capsules.

Exclusion Criteria:

• Presence of any medical condition that significantly increases risk or hampers assessment (e.g., unstable hypertension or cardiac disease, unstable asthma, kidney disease, unstable seizure disorder, pregnancy or any other medical condition as determined by the investigator).
• Weight < 15 kg.
• Subjects taking concomitant medications or supplements known for their glutamatergic effects (e.g., dextromethorphan, D-cycloserine, amantadine, memantine, lamotrigine, riluzole) or antioxidant properties (high dose vitamin supplements, DMG, TMG, many alternative treatments) within 30 days of the baseline visit with the exception of short term use of dextromethorphan as needed as a cough suppressant. The use of this medicine must be stopped at least 7 days prior to the baseline visit. Regular multivitamins will be allowed.
• Subjects taking daily acetaminophen or nonsteroidal anti-inflammatory drugs within 30 days of the baseline visit.
• Profound mental retardation as evidenced by a mental age below 18 months.
• Subjects taking concomitant medications with the potential for pharmacokinetic or pharmacodynamic drug-drug interactions (e.g., carbamazepine) within 30 days of the baseline visit.
• Subjects who are likely to experience significant changes in their ongoing psychosocial or medical treatments for autism over the course of the trial (e.g., initiation of new behavioral therapy, initiation of new medication or alternative treatment [e.g., chelation]). Minor changes in ongoing treatment (e.g., missed therapy sessions due to holiday/vacation; planned break in therapy due to school holidays) will not be considered significant.
• History of prior treatment with NAC.
• Evidence of hypersensitivity/allergy to NAC.
• Presence of certain neurodevelopmental disorders such as Fragile X Syndrome, Tuberous Sclerosis, or other neurological disorders known to be associated with autism or autistic features.
• Diagnosis of Rett's disorder, childhood disintegrative disorder, schizophrenia, bipolar disorder, another psychotic disorder, or substance abuse disorder.