The IPPO is a behaviour rating scale, developed for a more objective observation of psychiatric inpatients. Although there are several well-known rating scales, as for instance the NOSIE, the IPPO does have some advantages that are of specific value. Next to a high interrator-reliability and the small amount of time needed by the nursing staff to fill in the form, the scale can be used on different wards. The IPPO provides information about `Overall Level of Functioning' (Total Score), `Social Behaviour and Interest' (Subscale I) and `Awareness of Social Rules and Self Control (Subscale 2). The article presents the results of factor analyses, a try-out on 165 inpatients and several statistical investigations. There is sufficient material to conclude that the IPPO can be used as a reliable rating scale in the process of following the changes of behaviour of inpatients. In this way, the IPPO can be a valuable contribution to already known and used diagnostic methods.