BACKGROUND:The majority of individuals with fragile X syndrome (FXS) have intellectual disability, behavioral problems, autism, and language deficits. IQ typically declines with age in boys with the full mutation. The results of preclinical studies demonstrated that metformin, a biguanide used to treat type 2 diabetes, rescues multiple phenotypes of FXS in both Drosophila and mouse models. Preliminary studies of patients with FXS demonstrated improvements in behavior. METHODS:Here, we present two cases of individuals who have been treated with metformin clinically for one year. RESULTS:Both patients demonstrated significant cognitive and behavioral improvements. They also improved eating habits and normalization of their weight percentiles. CONCLUSION:Metformin may be a candidate drug for treatment of several types of symptoms in individuals with FXS.