Quetiapine, an atypical antipsychotic, is effective for psychosis in younger patients, with limited adverse effects reported. This open-label naturalistic study was conducted to assess the 4-week efficacy and safety of quetiapine for treatment of geriatric psychosis. Clinical efficacy was evaluated using the Brief Psychiatric Rating Scale (BPRS) and Clinical Global Impression Improvement (CGI-I) instruments before and after 4 weeks of quetiapine treatment. The sample population consisted of 100 geropsychiatric inpatients with psychosis, with the therapeutic evaluation completed by 91. Eighty-one of these 91 patients (89.0%) experienced mild-to-substantial improvement, as determined from the CGI-I. Further, a mean reduction in BPRS score of 39.5% (from baseline) was also determined. The mean daily dose of quetiapine for the fourth week was 276.1 177.2mg/day (range 50-800). Higher quetiapine dosages were administered for patients with functional psychoses compared to an analogous group with organic mental disorders. The most common adverse effects were somnolence (30.0%), lower-limb weakness (28.0%) and dizziness (27.0%). Body weight and fasting triglyceride were significantly elevated after quetiapine treatment (2.2% and 8.9% from baseline, respectively). Based on the results of this study, it appears that quetiapine is an efficacious and safe treatment for geriatric inpatients with psychosis, however, there is a wide dosing range and optimal dosage is diagnosis-dependent.