This randomized, double-blind, placebo-controlled, parallel-arm study will evaluate the safety and exploratory efficacy and pharmacokinetics of RO4917523 in pediatric patients with fragile X syndrome. Patients will be randomized to receive one of 2 dose levels of RO4917523 or placebo orally daily for 12 weeks.

Inclusion Criteria:

• Children and adolescents, 5 to 13 years of age
• Diagnosis of fragile X syndrome based on prior DNA testing confirming Fragile X Mental Retardation 1 (FMR1) full mutation and qualifying scores on the ABC and CGI-S

Exclusion Criteria:

• Previous treatment with another mGlu5 receptor antagonist within the prior 3 months
• Participation in a clinical trial involving an investigational drug (unapproved) or non-drug treatment within the prior 6 weeks or 5 times the half-life (whichever is longer) before the start of this study
• Any uncontrolled, unstable clinically significant psychiatric condition other than fragile X syndrome
• History of suicidal behavior
• Other protocol defined inclusion/exclusion criteria may apply