There is considerable controversy regarding whether persistent postconcussive symptoms (PCS) are injury-specific, in a subgroup of individuals after mild traumatic brain injury (TBI). The following findings have contributed to this controversy: (1) The base rate of PCS in mild TBI is comparable to uninjured controls; and (2) The severity of PCS covary with daily stress levels (Gouvier, Cubic, Jones, Brantley, & Cutlip, 1992). We examined this relationship further by evaluating the effects of experimental conditions of stress or relaxation on individuals with TBI and uninjured control subjects, with low and high PCS endorsement. We evaluated psychophysiological parameters, neuropsychological performance, and changes in PCS and stress perception. In our study, subjects with TBI increased PCS after engaging in cognitively challenging tasks, and demonstrated significant autonomic changes in the stress condition. Symptomatic TBI subjects exposed to high stress had increases in PCS complaints, decreased speed of information processing, and subtle memory deficits. Our results suggest that PCS are injury-specific and that individuals with a history of TBI are susceptible to the effects of stress. Relaxation training including breathing retraining may be an effective means of decreasing PCS and cognitive complaints in subjects with mild TBI.