The Mattis Dementia Rating Scale (MDRS) was developed to determine the degree of cortical deficit in those individuals with extremely low scores on the Wechsler Adult Intelligence Scale (WAIS) and the Wechsler Memory Scale (WMS). The purpose of this study was to potentiate the usefulness of this rating scale by determining its internal reliability when applied to patients with diffuse cerebral impairment. Subjects, ranging in age from 65 to 94 years, were chosen on the basis of their presenting an organic brain syndrome wtihout obvious localizing features nor evidence of reversible etiology. Responses to the MDRS were scored on split-half forms. Analysis of the scores using the Rulon formula for split-half reliability yielded a correlation coefficient of .90, indicating that the MDRS is very consistent internally. In addition, the MDRS is shown to be a useful test for discriminating the degree of impairment in individuals with organic mental syndrome.