This double-blind randomized study examined the effect of quetiapine (QTP) on drug-induced psychosis (DIP) in Parkinson's disease (PD). Conventional antipsychotic drugs are associated with adverse extrapyramidal effects. QTP is a new atypical antipsychotic drug used in the treatment of psychosis in PD. A total of 58 consecutive psychotic PD patients (mean age, 75 +/- 8.3 years; mean disease duration, 10.5 +/- 6.4 years; 29 with dementia) were randomly assigned to 2 groups: 30 were treated with QTP (mean dose, 119.2 +/- 56.4 mg) and 28 received placebo for 3 months. The motor part of the Unified Parkinson's Disease Rating Scale, the Brief Psychiatric Rating Scale, the Mini-Mental State Examination, the Hamilton Rating Scale for Depression, the Epworth Sleepiness Score, and the Clinical Global Impression Scale were administered before and during the study. No significant difference was found between the groups in all parameters. There were 32 PD patients (55%) completed the 3-month study (15 [26%] QTP and 17 [29%] placebo). Treatment was interrupted in 15 patients in the QTP and 11 in the placebo groups. This double-blind study did not show a beneficial effect of QTP for the treatment of DIP in PD. The high rate of withdrawal probably influenced the results. Larger double-blind studies are required.