The postconcussion syndrome is a term invoked to describe a constellation of cognitive (decreased memory and concentration), emotional (increased irritability and nervousness or anxiety), and physical (increased headaches and dizziness) changes that are commonly reported following minor or severe head injuries. The first section presents the development and validation of the Postconcussion Syndrome Checklist. The second section describes the daily changes in symptom reports as a function of daily stress levels. A population of head-injured subjects and controls monitored postconcussion symptoms and stress across a 6-week period. The frequency, intensity, and duration of symptoms reported were correlated with daily stress levels as measured by the Daily Stress Inventory for both groups. Yet, the number of symptoms reported by our head-injured group did not vary significantly from the normal subjects. The data support models which predict that postconcussion syndrome varies with stress, but the evidence for a reduction in the cerebral reserve capacity after head injury was not supported with this population. Explanations for this finding are presented and implications of this research and directions for future research are outlined.