This study will investigate the safety and efficacy of quetiapine in sub-optimally responding patients with DSM-IV schizophrenia using a double blind, randomized 12-week trial comparing oral doses of 1200 mg/d to 600 mg/d of quetiapine.

The study will be conducted at two sites: Manhattan Psychiatric Center (MPC), and the Clinical Research and Evaluation Facility at the Nathan S. Kline Institute for Psychiatric Research/Rockland Psychiatric Center (NKI). A total of 60 patients will be enrolled, 30 at each location. After a screening period of 1 week, all patients will be entering an open label, four-week quetiapine treatment period (run-in phase), during which quetiapine will be titrated to 600 mg PO daily and other adjunctive antipsychotics will be gradually tapered and discontinued. Other concomitant medications such as mood stabilizers will be maintained, if their dose has been stable for the preceding 2 months. Patients not responding to quetiapine treatment at 600 mg PO (defined as reduction of < 15% change in Positive and Negative Syndrome Scale (PANSS) total score between start of run-in to end of week 4) during the run-in phase, will be eligible to enter the double blind phase. Study baseline will be Day 7 of Week 4 of the run-in phase. Patients qualifying for the double-blind phase will be randomly assigned to either high dose 1200 mg quetiapine daily (Group A) or to 600 mg quetiapine (Group B) daily and treated on the assigned dose in a double blind fashion for 8 weeks (Week 1 through Week 8 of double blind phase). Measures of extra-pyramidal side effects, psychopathology, and safety will be conducted throughout the trial.

Inclusion Criteria:

1. DSM-IV criteria for chronic schizophrenia or schizoaffective disorder
2. Sub-optimal treatment-response
3. Total score > 60 on the Positive and Negative Syndrome Scale (PANSS) at baseline of run-in phase and again at baseline of double blind phase
4. Age 18-64 years old
5. Signed informed consent
6. Patient is in good general medical health

Exclusion criteria:

1. History of failure to respond to 400 mg/day or more of clozapine for 8 contiguous weeks
2. History of failure to respond to quetiapine treatment at dosages > 1200 mg daily for 6 contiguous weeks
3. History of quetiapine intolerance (e.g., clinically significant leukopenia or agranulocytosis, or severe dystonic reactions)
4. Significant recent history of violence or suicidal activity, which required > 4 episodes of PRN anti-agitation medication per week
5. Mental retardation
6. Depot antipsychotic within 30 days before randomization
7. Significant medical illness requiring frequent dose adjustment or medication changes

Clozapine non-responders are explicitly excluded, as they would be unlikely to show a response in this study.