Delirium is a common and serious complication of acute illness in elderly patients. The aim of this study was to develop and validate a model for predicting development of delirium in elderly medical inpatients who did not have delirium on admission. Consecutive admissions to an acute geriatric unit underwent standardized cognitive assessment every 48 hours. Delirium was diagnosed according to DSM-3 criteria. Independent predictors of delirium in a derivation group of 100 patients were determined using stepwise logistic regression analysis; the predictive model comprised dementia, severe illness and elevated serum urea. This model performed well in a validation group of 84 patients. We conclude that elderly medical patients can be stratified according to their risk for developing delirium using a simple clinical model.