In four Dutch nursing homes the reliability and validity of the behaviour observation scale 'Observation Psychogeriatrics' was investigated. This instrument was meant to assess the remaining capacity and need of care of psychogeriatric patients. It includes nine subscales: for ADL, mobility, specific daily activities, incontinence, time, place and person orientation, social contact, mood and disturbing behaviour, care by the inhabitant for others, care by relatives. The instrument is scored for 159 patients. All nine scales show high internal consistency. Interrater reliability is acceptable. No differences in this respect are found between the four nursing homes. Item reliability is sufficient in general. The scale is well capable of distinguishing psychogeriatric patients at different levels of impairment, especially in the less invalidated patients. The subscales that contribute to this classification, are the Orientation and Contact scale, as well as the scales for Activities of Daily Living and Instrumental Activities of Daily Living.