The coexistence of posttraumatic stress disorder (PTSD) and traumatic head or brain injury (TBI) in the same individual has been proposed to be paradoxical. It has been argued that individuals who sustain a TBI and have no conscious memory of their trauma will not experience fear, helplessness and horror during the trauma, nor will they develop reexperiencing symptoms or establish the negative associations that underlie avoidance symptoms. However, single case reports and incidence studies suggest that PTSD can be diagnosed following TBI. We highlight critical issues in assessment, definitions, and research methods, and propose two possible resolutions of the paradox. One resolution focuses on ambiguity in the criteria for diagnosing PTSD. The other involves accepting that TBI patients do experience similar symptoms to other PTSD patients, but that there are crucial differences in symptom content. (JINS, 2003, 9, 663–676.)