Background It has been suggested that women with traumatic brain injury have more favorable outcomes than do men because of higher levels of circulating estrogen and progesterone that may reduce brain edema. Objectives To determine whether there is any association between sex and mortality in TBI patients and whether there is any association between sex and brain edema. Design Retrospective cohort study using data from 2001 to 2007 collected from a trauma registry in Hong Kong and the Victorian State Trauma Registry. Setting Two regional trauma centers in Hong Kong and 2 adult major trauma centers and 1 pediatric trauma center in Victoria, Australia. Main Outcome Measures Mortality and brain edema. Patients Trauma patients with an Abbreviated Injury Scale score (head) of at least 3 who were aged 12 to 45 years were included. Patients with minor head injury and undisplaced closed skull fracture were excluded. Results Both the Hong Kong and Victorian data showed no significant difference in sex-related mortality. Increased mortality was associated with decreased systolic blood pressure and Glasgow Coma Scale score and with increased New Injury Severity Score or Injury Severity Score. In Hong Kong, brain edema was associated with female sex (P = .02), and the odds of brain edema in females were greater than for males. However, this association was not found in Victorian patients. Conclusion This study found no significant association between sex and mortality in either Victoria or Hong Kong and does not support the concept that females have better outcomes after traumatic brain injury.