OBJECTIVE The availability of magnetic resonance (MR) imaging data obtained in comatose patients after head injury is scarce, because MR imaging is somewhat cumbersome to perform in patients requiring ventilation and because, in the first hours after injury, its relevance is clearly inferior to computerized tomography (CT) scanning. The authors assessed the value of MR imaging in the early postinjury period. METHODS In this prospective study MR imaging was performed in 61 consecutive patients within 7 days after they suffered a severe head injury. An initial CT scan had already been obtained. To understand the clinical significance of the lesions whose morphological appearance was identified with MR imaging, brainstem function was assessed by registration of somatosensory and auditory evoked potentials. Brainstem lesions were visualized in 39 patients (64%). Bilateral pontine lesions proved to be 100% fatal and nonbrainstem lesions carried a mortality rate of 9%. In singular cases circumstances allowed for a clear clinical distinction between primary and secondary brainstem lesions. On MR imaging all lesions were hyper- and hypointense after intervals longer than 2 days. Within shorter intervals (< 2 days) after the injury, primary lesions appeared isointense on MR imaging. In one secondary brainstem lesion there were no traces of blood. CONCLUSIONS Because mean intracranial pressure (ICP) levels in patients without brainstem lesions were similar to those in patients with brainstem lesions, the authors conclude that it was not mainly increased ICP that accounted for the high mortality rates in patients with brainstem lesions. The authors also conclude that brainstem lesions are more frequently found in severe head injury than previously reported in studies based on neuropathological or CT scanning data. Early MR imaging after head injury has a higher predictive value than CT scanning.