In 1999 an Italian version of the AEP was applied to some units of the IRCCS Ospedale Maggiore di Milano and a modified AEP version was used to evaluate the day hospital activity. The main aims of the study were: to establish the appropriateness of the utilisation of beds and of day hospital activity and to calculate the costing of the misuse of hospital resources. 267 admission days were evaluated and 31.5% were not appropriated; whereas of 3,248 days of stay 37% were found not appropriated. The day hospital activity was evaluated through 300 record referred to 570 days of treatment and 13% of them were not appropriated. The cost of the 1,201 not appropriated hospital days was over 351.000. The not appropriated level of hospital utilisation in the IRCCS Ospedale Maggiore di Milano is similar to what is reported in national and international literature. Our study highlights some limits in the application of the AEP protocol at high specialised hospital units.