Primary objective: Given that poor effort may have a profound impact upon the results of neuropsychological assessment, it is of critical importance that assessment of effort is incorporated into the interpretation of a patient's neuropsychological profile. The current study examines the relationship between head injury severity ( as measured by length of post-traumatic amnesia) and level of cognitive functioning in patients passing or failing a test of effort ( Test of Malingered Memory). Research design: A cross-sectional correlational design was employed in a consecutive cohort of 78 persons with ongoing compensation claims. Methods and procedures: Head injury severity was assessed by duration of post-traumatic amnesia. All participants received a clinical interview followed by neuropsychological assessment. Main outcomes and results: A systematic and interpretable negative correlation between head injury severity and intellectual and memory functioning was demonstrated in persons passing the test of effort. However, in persons failing the test of effort no such relationships were demonstrated. Conclusions: The implications for the interpretation of neuropsychological test data are discussed. It is strongly recommend that valid interpretation of neuropsychological assessment data should include consideration of at least one test of effort.