Objective:The objective of this study was to investigate the interrater reliability of the Agitated Behavior Scale.Design:Ratings made by research assistants and nursing staff were compared.Participants and setting:Forty-five persons with brain injury and 23 persons with progressive dementia were studied at an acute rehabilitation unit and a long-term-care facility.Results:Ratings of persons with brain injury by research assistants yielded a correlation coefficient for the Total score of .920. The correlation coefficients for the factors Disinhibition, Aggression, and Lability were .902, .909, and .726, respectively. Lower coefficients were obtained when the ratings of the research assistants and nursing staff were correlated; these ranged from .364 to .604. The ratings by research assistants of long-term-care facility residents yielded coefficients ranging from .860 to .906 for the Total and factor scores.Conclusion:This study shows that the Agitated Behavior Scale is a reliable instrument for measuring agitation in persons with traumatic brain injury, as well as with long-term-care facility residents experiencing dementia.