To test the strength of the evidence in favor of the hypothesis that protein-energy undernutrition is an independent risk factor for non-elective hospital readmission within 3 months of discharge in a population of elderly hospitalized patients. Retrospective analysis of data from prospective observational study. All 110 elderly patients admitted to a geriatric recuperative care and rehabilitation unit during a 6-month period completed a comprehensive in-patient evaluation. Ninety-eight of these patients were subsequently discharged alive and followed prospectively for 3 months. All hospital readmissions during the observation period were identified by patient interview and, within the VA hospital system, computer tracking of admissions. Based on the discharge assessment, the strongest predictors of non-elective readmission were identified using univariate and multivariate statistical procedures. Twenty-eight of the 98 patients discharged alive and completing the 3-month follow-up (29%) had at least one non-elective readmission. The patients discharged home were non-electively readmitted more frequently than were the patients discharged to a nursing home (32% vs 11%, P = 0.05). Of the 109 discharge assessment variables analyzed, the best predictor of which patients would have at least one non-elective hospital readmission was the discharge serum albumin, followed by a diagnosis of dementia, discharge gamma globulin, the subscapular skinfold thickness, home ownership, and the discharge Katz Index of ADL score. Discharge serum albumin concentration, subscapular skinfold thickness, and discharge serum gamma globulin concentration were all negatively correlated with risk of non-elective readmission. The presence of functional debilitation or dementia was associated with a lower likelihood of non-elective readmission compared with the absence of these conditions. Protein-energy undernutrition appears to be a strong independent risk factor for non-elective hospital readmission especially among the highest risk patients, those who are functionally independent and cognitively intact.