To examine the reliability and validity of the Aggressive Behavior Scale (ABS), derived from the Minimum Data Set (MDS 2.0). Retrospective analysis of MDS 2.0 and Cohen Mansfield Agitation Inventory (CMAI) data. Ontario nursing homes (NHs) and complex continuing care (CCC) hospitals and units. Two hundred fourteen patients of a CCC hospital, 652 residents of four NH facilities who adopted the MDS 2.0 before its mandatory implementation, 124,259 CCC patients assessed with the MDS 2.0 between July 1996 and October 2006. In all samples, trained facility clinical staff completed the MDS 2.0 as part of normal clinical practice. The ABS is a 4-item summary scale measuring verbal and physical abuse, socially inappropriate behavior, and resisting care. In the single CCC facility, clinical facility staff completed the CMAI during the same assessment period as the MDS 2.0. Alphas for the ABS were between 0.79 and 0.93 for the three samples. A strong relationship was found between the ABS and the aggressive subscale of the CMAI (correlation coefficient=0.72, P<.001). Impairment in cognition was found to be related to higher ABS scores in all three samples. In CCC, individuals who had higher ABS scores also had a higher prevalence of psychiatric diagnoses and greater frequency of daily restraint use (P<.001 for each dependent variable). The ABS provides a useful measure of the severity of aggressive behavior that can be used for care planning, quality measurement, and research.