Organic delusional disorder (ODD) is rarely diagnosed in psychiatric in-patients, and may be misdiagnosed as delusional disorder (DD) from a similar clinical presentation. The aim of the present study was to investigate the characteristics of ODD and to make a comparison with those of DD patients. Patients who conformed to DSM-III-R criteria for ODD were recruited from an 8-year psychiatric in-patient database. Matching controls were DD patients admitted over the same time period. The prevalence of ODD according to DSM-III-R criteria was 0.4% of total admissions and 2.9% of organic mental disorders. Compared to DD patients, ODD patients less often had a family psychiatric history, and had an older age of onset of psychiatric disorder, longer hospital stays and lower treatment dosage of antipsychotic drugs. It is suggested that a detailed medical history and examination are needed in patients with delusion, especially in patients with a late onset of psychiatric symptoms and no family psychiatric history.