Psychosis induced by the standard drugs for treating idiopathic Parkinson's disease (PD) occurs, in the long term, in approximately 5-8% of patients. Until the development of atypical antipsychotic drugs, treatment of this psychosis was extremely limited and unsatisfactory. Clozapine has been reported to be an effective and well-tolerated treatment for this problem. Clozapine, however, is a difficult drug to use as a result of the monitoring system mandated by the Food and Drug Administration because of the potential for agranulocytosis. Treatment with risperidone has been shown to be poorly-tolerated. This article reports on the authors' open label experience with the newest atypical antipsychotic, olanzapine, and the switching of stable patients from clozapine to olanzapine. Nine of twelve subjects were unable to make the transition because of worsened parkinsonism. Most subjects preferred taking clozapine, despite the onerous monitoring procedure.