Objective: To assess the phenomenology and predictive factors of personality change due to traumatic brain injury. Method: Children (N = 177), aged 5 to 14 years with traumatic brain injury from consecutive admissions to five trauma centers, were followed prospectively at baseline and 6 months with semistructured psychiatric interviews. Injury severity, lesion characteristics, and preinjury variables including psychiatric disorder, family psychiatric history, family function, socioeconomic status, psychosocial adversity, and adaptive function were assessed with standardized instruments. Results: Personality change occurred in 22% of participants in the first 6 months after injury. Severity of injury predicted personality change, whereas none of the psychosocial variables predicted personality change. Lesions of the dorsal prefrontal cortex, specifically the superior frontal gyrus, were associated with personality change after controlling for severity of injury or the presence of other lesions. Conclusions: These findings are consistent with models of affective dysregulation that implicate a dorsal prefrontal cortex system important for effortful regulation of affective states. J. Am. Acad. Child Adolesc. Psychiatry, 2005;44(5):434-442. Key Words: traumatic brain injury, personality change disorder, frontal lobe, affective lability.