Our objective was to identify patient characteristics associated with inappropriate hospital days in a cohort of elderly medical inpatients. This prospective cohort study included a total of 196 patients aged 75 years and older, who were consecutively admitted over eight months to the internal medicine service of a regional, non-academic public hospital located in a rural area of Western Switzerland. Patients with severe cognitive impairment, terminal disease, or previously living in a nursing home were excluded. Data on demographics, medical, physical, social and mental status were collected at admission. A blinded hospitalization review was performed concurrently using a modified version of the Appropriateness Evaluation Protocol (AEP). Subjects' mean age was 82.4 years; 63.3% were women. Median length of stay was 8 days. Overall, 68 patients (34.7%) had at least one inappropriate day during their stay, including 18 patients (9.2%) whose hospital admission and entire stay were considered inappropriate. Most inappropriate days were due to discharge delays (87.10%), primarily to nursing homes (59.30%). Univariate analysis showed that subjects with inappropriate days were more likely to be living alone (69.1 vs 48.4%, p=0.006), and receiving formal in-home help (48.5 vs 32.8%, p=0.031). In addition, they were more impaired in basic and instrumental activities of daily living (BADLs, and IADLs, p<0.001 and p=0.015, respectively), and more frequently had a depressed mood [29.4 vs 10.9%, p=0.001 with a score > 6 at the Geriatric Depression Scale (GDS), short form]. Using multivariate analysis, independent associations remained for patients living alone (OR 2.6, 95%CI 1.2-5.8, p=0.016), those with a depressed mood (OR 2.8, 95%CI 1.1-7.3, p=0.032), with BADL dependencies (OR 1.5, 95%CI 1.2-1.8, p=0.001), and IADL dependencies (OR 1.3, 95%CI 1.0-1.6, p=0.032). Cardiovascular (OR 0.2, 95%CI 0.1-0.7, p=0.008) and pulmonary admission diagnoses (OR 0.1, 95%CI 0.0-0.7, p=0.022) were inversely associated with inappropriate hospital days. In conclusion, patients living alone, functionally impaired and showing depressive symptoms were at increased risk for inappropriate hospital days. These characteristics might permit better targeting for early discharge planning in these at-risk subjects, and contribute to avoiding premature discharge of other vulnerable elderly patients. Whether these interventions for at-risk patients will also result in prevention of hospitalization hazards, such as deconditioning and related functional decline, will require further study.