Glutamatergic dysfunction is implicated in the pathophysiology of fragile X syndrome (FXS). We report on the first trial of acamprosate, a drug with putative mGluR5 antagonism, in three adults with FXS and autism. Medical records describing open-label treatment with acamprosate in 3 patients with FXS and a comorbid diagnosis of autistic disorder were reviewed. In all three patients, acamprosate was associated with improved linguistic communication. Three patients received acamprosate over a mean 21.3 weeks of treatment. All patients showed global clinical benefit as rated with the Clinical Global Impressions-Improvement scale. Marked communication improvement was unexpected and has potential implications for the treatment of FXS, as well as idiopathic autism.