This randomized, multi-center, double-blind, parallel-group, placebo-controlled study will evaluate the efficacy and safety of RO4917838 in patients with sub-optimally controlled symptoms of schizophrenia. Patients, on stable treatment with antipsychotics, will be randomized to receive daily oral doses of RO4917838 or matching placebo for 52 weeks, followed by an optional treatment extension for up to 3 years.

Inclusion Criteria:

• Adult patients, >/= 18 years of age
• Diagnosis of schizophrenia
• Clinical stability for 16 weeks (4 months) prior to randomization
• Antipsychotic treatment stability for the past 12 weeks prior to randomization
• With the exception of clozapine, patients are on any of the available marketed atypical or typical antipsychotic (treatment with a maximum of two antipsychotics)

Exclusion Criteria:

• Has treatment resistant schizophrenia as judged by the treating physician OR have failed two trials
• Evidence that patient has clinically significant uncontrolled or unstable medical disorder (e.g. cardiovascular, renal hepatic, gastrointestinal, hematologic, immunological, neurological, endocrine, metabolic or pulmonary disease)
• Patient has a body mass index (BMI) of <17 or >40 kg/m2, respectively
• Diagnosis of mental retardation or severe organic brain syndromes
• In the investigator's judgment, a significant risk of suicide or violent behavior