A number of studies have shown that decompressive craniectomy can reduce intracranial pressure and may improve outcome for patients with severe head injury. This cohort study assessed the long-term outcome of neurotrauma patients who had a decompressive craniectomy for severe head injury in Western Australia between 2004 and 2008. The web-based outcome prediction model developed by the CRASH trial collaborators was applied to the cohort. Predicted outcome and observed outcome were compared. Characteristics of outcome between those who had had a unilateral and those who had had a bilateral decompressive procedure were compared. All complications were recorded. Among a total of 1,786 adult neurotrauma patients admitted during the study period, 147 patients (8.2%) had a decompressive craniectomy. A significant proportion of patients who required unilateral (37.3%) and bilateral (46.5%) craniectomy were able to return to work or study at 18 months after the injury. The patients who required bilateral craniectomy more likely to be associated with an unfavorable outcome (Glasgow Outcome Scale score >or=3) than those who had unilateral craniectomy (odds ratio 4.42; 95% confidence interval 1.16,16.81; p = 0.029), after adjusting for the timing of surgery, mechanism of injury, and the predicted risk of unfavorable outcome. The functional outcome after either unilateral or bilateral decompressive craniectomy was significantly better than that predicted by the CRASH head injury prediction model when the predicted risk was less than 80%. This study has demonstrated that in Western Australia decompressive craniectomy is a relatively common surgical procedure for the management of neurotrauma. A significant proportion of patients had a better-than-predicted long-term functional outcome.