['To identify potentially modifiable environmental factors (including number of medications) associated with changes over time in the severity of delirium symptoms and to explore the interactions between these factors and resident baseline vulnerability.', 'Prospective, observational cohort study.', 'Seven long-term care (LTC) facilities.', 'Two hundred seventy-two LTC residents aged 65 and older with and without delirium.', 'Weekly assessments (for up to 6\xa0months) of the severity of delirium symptoms using the Delirium Index (DI), environmental risk factors, and number of medications. Baseline vulnerability measures included a diagnosis of dementia and a delirium risk score. Associations between environmental factors, medications, and weekly changes in DI were analyzed using a general linear model with correlated errors.', 'Six potentially modifiable environmental factors predicted weekly changes in DI (absence of reading glasses, aids to orientation, family member, and glass of water and presence of bed rails and other restraints) as did the prescription of two or more new medications. Residents with dementia appeared to be more sensitive to the effects of these factors.'] Do environmental factors predict the severity of delirium symptoms in long-term care residents with and without delirium?