In this research study we want to understand the effectiveness of a drug treatment, acamprosate, for interfering symptoms (i.e., inattention/hyperactivity, social impairment) associated with Fragile X Syndrome (FXS).

Each subject with FXS will receive 10 weeks of blinded treatment with acamprosate or matching placebo. After completion of the double-blind phase, all subjects will have an opportunity to receive acamprosate as part of the study procedures for 16 weeks.

Inclusion Criteria:

• Diagnostic confirmation of full mutation FXS
• Age ≥5 years and <23 years
• General good health as determined by physical exam, medical history and laboratory work up.

Exclusion Criteria:

• Use of more than two psychotropic medications (medications affecting behavior).
• Unstable dosing of any psychotropic medication (medication affecting behavior)
• Problems with kidney functioning
• Unstable seizure disorder
• Change in any anti-convulsant drug dosing in the 60 days prior to study entry
• Prior adequate treatment trial with acamprosate as determined by the study doctor
• Pregnant or lactating females