Clozapine is the current treatment of choice for drug-induced psychosis (DIP) occurring in Parkinson's disease. However, alternative medications have been sought because of the small but significant risk of agranulocytosis and the need for frequent blood testing. The new "atypical" antipsychotic olanzapine (OLZ) has recently been proposed as a safe and effective option for treating psychosis in this setting. To investigate this, we retrospectively evaluated all 12 of our patients treated with OLZ for DIP. Symptoms of psychosis were improved in nine of 12 patients, but nine of 12 patients also experienced worsening of motor functioning while on OLZ. The worsening was considered dramatic in six of these patients. Overall, there was no significant increase in levodopa doses on OLZ. Only one patient remained on OLZ at the time of the analysis. Nine patients were switched to alternative treatment for DIP. We conclude that although OLZ may improve symptoms of psychosis in parkinsonian patients, it can also worsen motor functioning. In some patients, the degree of motor worsening may be intolerable.