ABSTRACT: Delirium is a common cause of functional and cognitive decline, morbidity, and mortality among hospitalized elderly individuals. Several studies reveal that the prognosis of delirium is worse among elderly individuals with severe delirium. In light of these findings, it is important to identify which factors are associated with delirium severity: individual (predisposing) or environmental (precipitating) factors. This study wanted to investigate individual and environmental factors associated with delirium severity among older persons with delirium superimposed on dementia. This study is a secondary analysis of a cross-sectional study (N = 71) on delirium carried out in three long-term care facilities and one long-term care unit of a large regional hospital. Of the 29 potential risk factors considered, researchers found 6 to be significantly associated with delirium severity in univariate analysis: marital status (being married), severity of dementia, lower functional autonomy, less medication consumption, presence of behavioral problems, and inadequacy of the physical environment. In multivariate analysis, only marital status (being married) and severity of dementia remained statistically associated with delirium severity. Results of this study provide further evidence that the weakened brain functions of persons with dementia increases not only the risk of delirium but also its severity. Given the poor outcomes associated with delirium severity, nurses need to pay closer attention to the predisposing and precipitating factors of delirium severity.