To assess the extent of inappropriate hospital use in an adult in-patients population we used a modified version of the Appropriateness Evaluation Protocol (A.E.P.) to evaluate retrospectively a cross-section of 273 patient-days in a large teaching hospital in the Greater Milan area. Overall, 41% were judged to represent inappropriate hospital use on the basis of the protocol's criteria. The rate of inappropriate hospital use was significantly associated with admitting specialty, ranging from 12% for surgery, to 20% for cardiology and to about 60% in psychiatric, geriatrics and neurology departments (p less than 0.01). Hospital days of patients with longer stays were more frequently inappropriate: a statistically significant trend of inappropriateness emerged ranging from 30% among patients with total length of stay (LOS) of 1-10 days to 60% among those with LOS greater than 30 days (p less than 0.01). This study confirms that there is a substantial rate of unnecessary use of hospitals but that such inappropriateness does not seem in most cases to be easily modifiable through "simple" organizational changes.