While schizophrenia affects an estimated 1% of the population, its treatment consumes approximately 2.5% of annual health care expenditures. Hospitalization of schizophrenic patients is often due to ineffectiveness of or noncompliance with pharmacological treatment. Clozapine is generally considered the most effective antipsychotic medication, but its use is limited due to the potential for serious side effects such as agranulocytosis, weight gain, seizures, and metabolic disturbances. The metabolism of clozapine is affected by individual patient factors, including ethnicity, smoking, and concomitant medications that induce or inhibit cytochrome P450 1A2. Widely variable plasma clozapine concentrations may lead to treatment failure or toxicity. These complications may be avoided if validated clozapine plasma concentrations were established and implemented in the stable outpatient population. This study will compare clozapine blood levels attained in a stable, outpatient population to individual response measured by a validated psychiatric rating scale.