The purpose of this study is to assess and compare how risperidone and olanzapine, two antipsychotic medications, affect the regulation of glucose (sugar) in the body.

The newer antipsychotic medications risperidone and olanzapine effectively treat schizophrenia and related disorders, and they may cause fewer side effects than the older antipsychotic drugs do. However, some of the newer antipsychotic medications could have a negative effect on the regulation of blood glucose (sugar) in the body. This negative effect could make the patient gain weight and even develop diabetes mellitus. This randomized, double-blind study will assess and compare how risperidone and olanzapine affect the regulation of glucose in the body in patients with schizophrenia and schizoaffective disorder. Patients will be randomly assigned to receive risperidone (target oral dose of 6 milligrams per day) or olanzapine (target oral dose of 20 milligrams per day) for 6 months. Repeat laboratory measurements will be performed and questions will be asked of the patients to evaluate the safety and effectiveness of the drugs. The primary laboratory test used to assess the patient's regulation of blood glucose will be the Disposition Index derived from the Frequently Sampled Intravenous Glucose Tolerance Test. Risperidone 2 milligram oven-encapsulated tablets taken orally once a day for 6 months; olanzapine 5 milligram tablets taken orally once a day for 6 months

Inclusion Criteria:

• Diagnosis of schizophrenia or related disorder
• stable with respect to disease symptoms and other medical conditions
• would benefit from this type of antipsychotic drug
• if female, using birth control.

Exclusion Criteria:

• Patients who are delirious, bipolar, severely mentally retarded, or suicidal
• psychiatric diagnosis of disease unrelated to schizophrenia
• presence of stroke, brain tumor, Parkinson's Disease, or diseases that affect blood glucose control
• history of diabetes
• long or recent history of taking risperidone, olanzapine, quetiapine, clozapine, or investigational drugs
• recent history of unstable thyroid function
• if female, not using birth control
• abusing drugs or alcohol.