This prospective study determined the incidence and prevalence of delirium in 235 consecutive subjects over age 70 admitted to a general medicine hospital service. The DSM-III criteria for delirium were operationalized. Using accepted screening procedures, patients were referred for evaluation by a psychiatrist who determined whether delirium was present by applying explicit operational definitions to each of the DSM-III criteria. Data on presence and severity of each of the DSM-III symptoms were recorded. Analysis of these data indicates that the DSM-III criteria describe a discrete, recognizable syndrome. However, some of the symptoms are more specific than others in identifying the syndrome in this population.