Clozapine is the prototype atypical antipsychotic indicated primarily for schizophrenic patients either refractory or intolerant to typical antipsychotics. Even for this patient population however clinical response to clozapine is highly variable. The effects of all antipsychotics on the symptoms of schizophrenia, including clozapine, are thought to be mediated, at least in part, through their interactions with the dopaminergic system via various dopamine receptor subtypes. Furthermore, twin case-studies provide evidence that there is a genetic basis for the observed inter-patient variability in antipsychotic response. Therefore, genetic variants which affect the function and/or density of dopamine receptors may help to explain some of this observed variability. This thesis presents association studies of dopamine D1 and D2 receptor polymorphisms on clozapine response after 6 months treatment in Caucasian and African American schizophrenic patient populations. Both categorical (responder/non-responder) and quantitative measures (% change score) of treatment response were used.