Aggression, including physical assault, is a significant problem in providing services for older people with mental health problems. A range of bio-psycho-social correlates of aggressive behaviour have been explored in this client group, but little attention has been given to the role of anger as an activator of aggression, despite its demonstrated predictive association with aggression in other clinical populations. In this pilot study, a staff-rated anger measure was administered to 27 inpatients in a specialist service for older people with mental health problems. The Anger Index was found to have high internal consistency and inter-rater reliability, and it showed robust concurrent and discriminant validity with comparison measures completed by independent raters. Higher anger scores were associated with organic diagnoses, history of aggression, and hospital assault data. The potential role of anger in the activation of aggression, the utility of anger assessment in the evaluation of risk, and the value of therapeutic approaches for aggression problems in older adult patients are discussed.