The purpose of the study is to assess and compare the side effect profile, safety, tolerability and efficacy of schizophrenic or schizoaffective subjects non- or partially- responsive to 800 mg/day of quetiapine treated with either 800 mg/day or more than 800 mg/day of quetiapine during 8 weeks.

Inclusion Criteria:

• Inclusion Criteria: Men and women, age 18-65 years with persistent positive or negative symptoms with a PANSS total score of ≥ 70 and < 110 and a Clinical Global Improvement (CGI)- Severity of Illness score of at least 4 (moderately ill) at screening. Subjects may be treated as in- or out-patients. Subjects must give written informed consent.

Exclusion Criteria:

• Subjects with other psychiatric, medical or behavioural comorbid disorder that may interfere with study conduct or interpretation.
• Female of childbearing potential, unless the subject is using a reliable method of contraception
• Subjects with alcohol or psychoactive-substance dependence not in full remission or with significant alcohol or substance abuse in the past 3 months will be excluded.
• Laboratory test results outside the range of reference considered by the investigator to be clinically significant.
• Inability to respect the visit schedule and known intolerance to quetiapine at 800mg/day.