Quantitative evaluation of sway as an indicator of functional balance in post-traumatic brain injury. The test of sway, using different conditions of stance with measurements of the average radial deviation of the center of pressure and its path length of sway per unit of time, has been shown to be a useful clinical tool in determining balance problems in traumatic brain injury (TBI) patients. Normative values were established to determine if an individual patient's sway values fell within the normal range (mean +/- 2SD). The tests have shown good test-retest reliability for TBI patients. In addition, it has been shown that the sensitivity of the test is sufficient to identify changes in patients' performances as their clinical conditions change. It has been demonstrated that the different stance conditions of the battery of tests become progressively more difficult to perform (from comfortable stance, eyes open and eyes closed, through narrow stance, eyes open and eyes closed, to tandem stance with right or left foot forward, eyes open and eyes closed). By using these subtests, it is easy to distinguish between the performances of able-bodied patients and TBI patients with very mild balance problems. The validity of the measure has been documented by correlating the sway performance with clinical functional performance tests. The test performance also correlates with the patient's own assessment of his or her gait difficulties. The limited data available suggest that the test of sway relates difficulties in static balance to the frequency of falls. Finally, subtests permit identification of specific problems in maintaining balance as a basis for therapeutic intervention. 