Athletic trainers and team physicians are often faced with the dilemma of when to return athletes to participation following mild head injury (MHI). Unfortunately, clinicians rarely have quantitative information on which to base their decision. The purpose of this investigation was to identify postural stability changes in acute MHI subjects. Seventy male athletes were pre-screened for postural stability and cognitive functioning. Postural stability testing involved the use of the Chattecx Balance System™ during three different eye conditions and three different support surface conditions. Cognitive testing involved the use of the Paced Auditory Serial Addition Task, which assessed central information processing. Ten of the pre-screened subjects suffered a MHI and returned for testing on days 1,3,5 and 10 following injury, and one month post-season. A group of matched control subjects was selected to compare between group differences. A repeated measures ANOVA for sway index (SI) revealed a significant interaction for group by day by platform. Significant differences were revealed between MHI subjects and control subjects on day 1 post-injury as compared to the pre-screening and/or subsequent tests. An additional analysis for dynamic accuracy scores (DAS) revealed a significant interaction for group by day. MHI subjects revealed significant improvements from day 1 post-injury to day 3 post-injury, however, control subjects demonstrated insignificant changes. Additionally, nine subjects having sustained MHI were recruited from local high schools and local emergency rooms. These subjects and matched controls were assessed under the same conditions as pre-screened subjects. The analysis for center of balance (COB) revealed a significant interaction for group by eyes by platform. During the moving platform condition, MHI subjects maintained their COB at a greater distance from the center of their base of support, especially when vision was confounded. Control subjects, however, moved closer to the center of their base of support as vision was confounded. An additional analysis for SI inclusive of all MHI subjects and all control subjects revealed a significant group R eproduced with perm ission of the copyright owner. Further reproduction prohibited without perm ission. by day by platform interaction. MHI subjects demonstrated increased postural sway compared to control subjects on day 1 post-injury during all platform conditions, and on day 3 during the foam platform condition. The analysis for COB using the same subjects revealed a significant group by day interaction, as MHI subjects again maintained their COB further away on day 1 post-injury compared to subsequent tests. These findings suggest that the Chattecx Balance System ™ is a useful tool in objectively assessing postural stability in MHI subjects. Subjects with MHI appear to demonstrate impaired postural stability 1 to 3 days following injury. This information should aid clinicians in making decisions related to determining when an adiletc can safely return to participation.