Quetiapine is an atypical antipsychotic with clozapine-like pharmacology but without associated agranulocytosis. We report our complete experience with quetiapine for the treatment of drug-induced psychosis (DIP) in Parkinson's disease (PD). Thirty-five patients with PD and DIP aged 75 years (range, 58-89) with a mean PD duration of 8.4 years on an average of 427 mg levodopa per day received a mean dose of 40.6 mg quetiapine daily. Twenty of 24 neuroleptic-naive patients reported marked improvement of psychosis without a decline in motor function as assessed by the Unified Parkinson's Disease Rating Scale (UPDRS-motor). Ten patients had a baseline and 4-week follow-up assessment using the Mini-Mental Status Examination (MMSE) and Brief Psychiatric Rating Scale (BPRS). The improvement in BPRS score (32.6 versus 22.8) was clinically and statistically significant (p = 0.024). Three of 24 were unable to tolerate quetiapine because of orthostatic hypotension, headache, nausea, and persistence of hallucinations. One patient died of an unrelated cause. We also tried to switch 11 psychiatrically stable patients on clozapine (eight) and olanzapine (three). Five patients made this transition without a loss of effect as measured on BPRS and MMSE. Six did not (five on clozapine, one on olanzapine) because of confusion, erratic behavior, and increased hallucinations. No crossover failure had worsened PD except for increased tremor in one. Quetiapine is useful and well-tolerated as a first drug to treat DIP in PD but must be used cautiously to replace other atypical antipsychotic drugs.