To assess patterns of regional brain activation in response to varying working memory loads shortly after mild traumatic brain injury (MTBI). Many individuals complain of memory difficulty shortly after MTBI. Memory performance in these individuals can be normal despite these complaints. Brain activation patterns in response to a working memory task (auditory n-back) were assessed with functional MRI in 12 MTBI patients within 1 month of their injury and in 11 healthy control subjects. Brain activation patterns differed between MTBI patients and control subjects in response to increasing working memory processing loads. Maximum intensity projections of statistical parametric maps in control subjects showed bifrontal and biparietal activation in response to a low processing load, with little additional increase in activation associated with the high load task. MTBI patients showed some activation during the low processing load task but significantly increased activation during the high load condition, particularly in the right parietal and right dorsolateral frontal regions. Task performance did not differ significantly between groups. MTBI patients differed from control subjects in activation pattern of working memory circuitry in response to different processing loads, despite similar task performance. This suggests that injury-related changes in ability to activate or to modulate working memory processing resources may underlie some of the memory complaints after MTBI.