Changes in balance ability can be a sensitive measure of post-concussion changes in brain function. However there is a need to determine the most reliable, valid and practical ways to assess balance abilities (i.e. degree of postural sway). The Sway Balance™ System is a mobile software application that uses the tri-axial accelerometer located within an iOS device to quantify thoracic sway. The software relies on comparing the individual’s post-concussion score to baseline scores obtained at the start of an athletic season. However, in youth athletes in particular, balance may change over the course of the season, worsening due to factors other than concussion, such as lower limb injuries, or improving as fitness improves. This study first compared baseline Sway Balance™ scores to the industry standard accelerometer data recorded from the sternum and attached externally to the iOS device in an elite youth (aged 16 to 20) lacrosse team. We found that balance scores derived from Sway Medical’s algorithm had moderate to very strong validity when compared to laboratory accelerometers. We then assessed the reproducibility of the Sway Balance™ System at 5 and 10 weeks relative to baseline. Correlations ranged from weak to excellent depending on which weeks were compared. Variations were observed in some of the athletes’ balance scores over the 10 weeks, and in overall group differences at different measurement points. This work indicates that while software such as the Sway Balance™ System has the validity to act as a balance measure, there is considerable work needed in protocol development to ensure that a change in score genuinely reflects a change in balance in concussed athletes.