The Agitated Behavior Scale (ABS; Corrigan, 1989) is a 14-item scale developed to monitor agitation during the acute phase of recovery from acquired brain injury. While previous studies have supported the reliability, internal consistency, and concurrent validity of the ABS, the current study was designed to investigate its underlying factor structure, as well as to determine systematic effects of time-of-day on the occurrence of agitation. Subjects were 212 patients with traumatic or other recently acquired brain injury who exhibited agitation during their treatment on a specialized brain-injury unit of an acute rehabilitation hospital. Confirmatory factor analysis revealed that agitation is best represented by one general construct with three underlying factors: Aggression, Disinhibition, and Lability. Analysis of agitation by nursing shift in which ratings were made confirmed that overall level was lowest during the night shift; however, the relationship between time-of-day and underlying factors deviated from this pattern. Results are discussed in terms of the necessity for objective measurement, definition of the construct of agitation, and time-of-day issues in sampling agitated behavior.