We investigated the usefulness of quetiapine and olanzapine for delusion in nine patients with Parkinson's disease (PD). Two of five patients initially treated with quetiapine showed resolution of delusion on low dose (25 mg and 50 mg/day), whereas three patients had no improvement in spite of increasing the dose to 300 mg/day. Quetiapine non-responders had a tendency to more severe delusion and dementia compared with responders. Not only all four patients initially treated but also two of three quetiapine non-responders showed a remarkable resolution of delusion on olanzapine. In particular, three of seven patients responded to an extremely low dose of 0.625 mg/day. However, severe motor debilitation was observed in one patient treated with 1.25 mg/day. Olanzapine might be useful for delusion of PD in patients not responded to quetiapine, although it should be started at a very low dosage to ameliorate worsening parkinsonism.