SUMMARYThis study compares a new psychogeriatric rating scale, the Organic Brain Syndrome (OBS) scale, which is a combined observation and interview scale, with two other scales. Twenty-eight elderly demented patients took part in the study. The OBS scale was compared to the Gottfries, BrPne and Steen (GBS) scale, the Mini Mental State Examination (MMSE) scale and a global rating. The correlations between OBS, GBS and MMSE were high, showing that the concurrent validity is good. The ratings of the OBS scale were independent of the patient's physical disabilities as measured by Katz's index, ie the discriminant validity was good. The analyses show similarities and differences between the clinical dimensions described in the rating scales. The OBS scale contributes unique clinical information and is a valid instrument for assessment of dementia symptoms.KEY woms-Dementia, confusion, disorientation, geriatric rating scales, interview, psychogeriatrics.The evaluation of psychopathology in elderly patients has to rely on valid and reliable clinical instruments. The rating scales should enable the investigator to describe a clinical picture in detail and follow a disease process in patients suffering from dementia, delirium, and other organic brain syndromes.A main purpose of the present study was to evaluate a psychogeriatric rating scale-the Organic Brain Syndrome (OBS) scale-by comparison with two other rating scales, the Mini Mental State Examination (MMSE) (Folstein et al., 1975) and the Gottfries, Brine and Steen (GBS) scale (Gottfries et al., 1982). The OBS scale was primarily devised for the description of patients with dementia and confusional states (Gustafson et al., 1985). It consists of two parts-one patient interview scale giving a short time perspective and one observation scale based on information from staff and caregivers giving a longer time perspective. Thus the OBS scale covers both reported symptoms and observed signs, and it contains more symptoms than can be expected from an interview or an observation scale only. The scale offers a broad description of the psychopathology of organic brain syndromes while also describing various fluctuations of the clinical state which often create a need for extra medical measures and nursing. The OBS scale covers not only primary cognitive dementia symptoms but also other emotional and conative symptoms which are frequently observed in organic brain syndromes but less frequently presented in the rating scales for these conditions.In addition to the MMSE and the GBS there exist many rating scales which could be suitable for evaluating a new rating scale, for instance the Sandoz Clinical Assessment Geriatric (SCAG) scale (Shader et al., 1974) and the Information/ Orientation (10) subtest of the Clifton Assessment Procedure (CAPE) (Pattie and Gilleard, 1979). But both the MMSE and the GBS are widely used and translated into many languages. In particular, there are routines for using these two rating scales in the Scandinavian area. Furthermore, GBS is a comprehe...