This is an open-label extension protocol that will provide necessary data on the safety, tolerability, pharmacokinetics and efficacy of STX209 among subjects with ASD.

This is an open label extension study enrolling by invitation only to those subjects that have completed Seaside protocols 209AS208 and 22007.

Protocol 209AS208, "A Randomized,Double-Blind, Placebo-Controlled, Study of the Efficacy, Safety, and Tolerability of STX209 (Arbaclofen) Administered for the Treatment of Social Withdrawal in Subjects with Autism Spectrum Disorders."

Protocol 22007, "An Open Label Extension Study to Evaluate the Safety, Tolerability and Pharmacokinetics of STX209 in Subjects with Autism Spectrum Disorder."

This open-label extension will provide data on the following:

1. Evaluate the safety and tolerability of long term use of STX209 (Arbaclofen)
2. Provide supporting pharmacokinetic analyses
3. Assess long term efficacy on social behaviors in subjects with ASD.

Inclusion Criteria:

• Subject completed study 22007 or 209AS208, and showed he can adequately follow the protocol, and with adequate medical justification to enter this study.
• Parent or other legally-authorized representative or caregiver is willing and able to perform all protocol-specified functions.
• Treatment with no more than 2 psychoactive medications
• Subjects with a history of seizure disorder must be adequately well-controlled, as specified in the study protocol
• For female subjects, negative pregnancy test

Exclusion Criteria:

• Comorbid conditions that might interfere with the conduct of the study or confound the interpretation of the study data, or endanger the subject.
• Current use of illicit drugs or alcohol abuse.
• Subjects with a serious adverse event or other adverse event in study 22007 or 209AS208 that was related to STX209, and that endangers the subject, in the opinion of the investigator
• Current use of another investigational drug, or of vigabatrin, tiagabine, or riluzole