The decision to discharge is an important factor determining the length of stay (LOS) in a rehabilitation center and should be scrutinized. The purpose of this study was to analyse the predictive power of the individual items of the admission functional independence measure (FIM) for the LOS indicating their relevance in the decision to discharge and to expose the assumptions driving this decision. The data of all consecutive in-patients of 5 years in a rehabilitation center were analysed (n=1047). The regression analysis of FIM item values on admission and FIM item gains as independent variables and the LOS as the dependent variable showed a number of criteria operational in the decision to discharge patients with different diagnoses. The criteria were identified as 'aiming for certain standards' (for example, bed/chair/WC transfer), 'aiming for optimal improvement', 'dealing with different rates of improvement' and 'giving benefit of learning potential'. It is proposed that these criteria should be discussed and evaluated.