This study will determine the efficacy and safety of RO4917838 in patients with schizophrenia who are stable on current antipsychotic treatment (olanzapine, que tiapine, risperidone, paliperidone or aripiprazole) with prominent negative or d isorganized thought symptoms. After a 4 week run in period on their current anti psychotic treatment, patients will be randomized to receive placebo 10mg, 30mg, or 60mg of RO4917838 once daily, p.o., as add-on therapy. The anticipated time o n study treatment is <3 months, and the target sample size is 100-500 individual s.

Inclusion Criteria:

• adult patients, 18-60 years of age;
• diagnosis of schizophrenia (based on screening tests);
• outpatients, with no hospitalization for worsening of schizophrenia within 3 months;
• medically and psychiatrically stable over prior 1 month and psychiatrically stable without symptom exacerbation within prior 3 months;
• currently taking no more than 2 antipsychotic drugs.

Exclusion Criteria:

• began a new non-medication treatment for schizophrenia or other psychiatric condition within last 3 months;
• on >1 antidepressant, or a change in dose of antidepressant within 3 months;
• alcohol or substance abuse or dependence within 3 months;
• has previously received RO4917838.