Recently, several small studies have indicated that quetiapine may be useful in the treatment of drug-induced psychosis in patients with PD. However, there have been questions related to atypical antipsychotic therapy and patient selection and how that may affect response and tolerability, especially worsening of the motor symptoms of PD. In particular, the presence or absence of dementia seems to be important. The aim of this study was to evaluate the effect of quetiapine on psychosis and motor symptoms in PD patients with and without dementia. A retrospective record review of patient responses to quetiapine was conducted. Response of psychosis was assessed through patient and caregiver interviews. Motor symptom change in relation to this therapy was assessed by patient and caregiver interviews and completion of the motor portion of the Unified Parkinson's Disease Rating Scale (UPDRSm). Analysis was performed by comparing psychosis and motor feature measures from before and after therapy for the group as a whole and for demented and nondemented subgroups, using nonparametric tests and Fisher's exact test. Forty-three consecutively treated PD patients were evaluated. The mean dose of quetiapine was 54 mg per day and the duration of therapy was 10 months. Eighty-one percent of patients demonstrated partial or complete amelioration of psychosis. Five patients (13%) experienced a worsening of PD motor symptoms, which was corroborated by changes seen in UPDRSm. When the group was examined as a whole, no significant change in UPDRSm score was noted. When demented (n = 20) and nondemented (n = 19) patients were compared, improvement in psychosis occurred in similar numbers of patients, but a significant difference in the numbers of patients who experienced a worsening of motor symptoms was seen (P < 0.02, Fisher's exact test). All five patients who complained of motor worsening were in the demented group. UPDRSm score after therapy tended to be worse in the demented group (P = 0.55, Mann-Whitney U test). There was no significant change in the levodopa dose. Approximately 80% of patients chose to continue therapy. We conclude that quetiapine is effective in improving psychosis in approximately 80% of PD patients both with and without dementia. Patients with dementia seem to have a higher propensity for worsening of motor symptoms.