OBJECTIVE: To determine the immediate effects of a whole-body fatigue protocol on performance of the Balance Error Scoring System (BESS), a postural-stability test commonly used as part of a concussion-assessment battery. DESIGN AND SETTING: Subjects were assigned to a fatigue or control group and were assessed before and immediately after a 20-minute fatigue protocol or rest period. SUBJECTS: Fourteen fatigue subjects and 13 control subjects participated in this study. All subjects were male and free of vestibular disorders, and none had suffered a mild head injury or lower extremity injury in the preceding 6 months, as described through self-report. MEASUREMENTS: We measured performance on the BESS for 9 stance-surface conditions and summed each condition to obtain a total score. Using the Borg scale, we also measured ratings of perceived exertion before, during, and after the fatigue protocol or rest period. RESULTS: We found a significant increase in total errors from pretest to posttest in the fatigue group (14.36 +/- 4.73 versus 16.93 +/- 4.32), a significant decrease in errors in the control group (13.32 +/- 3.77 versus 11.08 +/- 3.88), and a significant difference between groups on the posttest. The rating of perceived exertion scores were significantly different between the fatigue and control groups at the middle (13.29 +/- 1.59 versus 6.23 +/- 0.83) and end (15.86 +/- 2.38 versus 6.15 +/- 0.55) of the fatigue or rest period. CONCLUSIONS: The BESS error scores increased immediately after the fatigue protocol, demonstrating that balance ability diminished. Clinicians who use the BESS as part of their sideline assessment for concussion should not administer the test immediately after a concussion due to the effects of fatigue.