BACKGROUND Reduction of cerebral blood flow plays a crucial role in causing posttraumatic cerebral ischemia. However, the methodologic adequacy of studies from which currently used cerebral blood flow thresholds in traumatic brain injury have been derived has not been evaluated. OBJECTIVE To systematically evaluate the evidence available on cerebral blood flow thresholds and its methodologic adequacy in adults with traumatic brain injury. METHODS Included were primary studies on adults with traumatic brain injury in which cerebral blood flow thresholds were evaluated and reported, and follow-up brain computed tomography or magnetic resonance imaging was used as the gold standard for diagnosing the finally infarcted area. RESULTS Among the 53 diagnostic studies identified, 31 did not report any threshold value, whereas 20 studies used thresholds derived from the literature, mainly animal or clinical studies on ischemic stroke. One study measured cerebral blood flow thresholds, but did not use accepted neuroradiological criteria for the diagnosis of posttraumatic cerebral ischemia. The remaining study fulfilled all methodologic inclusion criteria, but was restricted to 14 patients with severe traumatic brain injury and cerebral contusion. This study proposed a cerebral blood flow threshold of 15 mL/100 mL/min, with sensitivity and specificity of 43% and 95%, respectively. CONCLUSIONS Cerebral blood flow thresholds for the diagnosis of posttraumatic cerebral ischemia are based on weak evidence, and cannot be recommended.