Organic delusions are common, but have received little systematic study. Review of the literature reveals that they occur most commonly in toxic-metabolic processes and in disorders affecting the limbic system and basal ganglia. A prospective study of 20 consecutive patients with organic delusions revealed four general types of false beliefs: simple persecutory delusions, complex persecutory delusions, grandiose delusions, and those associated with specific neurological defects (anosognosia, reduplicative paramnesia). Simple delusions responded best to treatment, and complex delusions were more resistent. Acting on delusional beliefs was not unusual, and treatment of the delusions was an important aspect of management of the patient.