Objective To test a proposed mechanism for the effect of Balance-Based Torso-Weighting (BBTW) in people with multiple sclerosis and healthy controls. The application of light weights to the trunk may result in a biomechanical shift of postural sway in the direction of weighting, mechanically facilitating maintenance of center of mass over the base of support. Design Non-randomized controlled trial Setting motion analysis laboratory Participants 20 participants with multiple sclerosis (average EDSS 4.1) and 18 sex-, age-, height- and weight-matched controls Intervention Light weights strategically placed according to the BBTW protocol were applied to all participants following at least three walking trials and 10 seconds of quiet standing with feet together eyes open (EO) and eyes closed (EC). Measures were repeated after weighting. Main Outcome Measure Force plate center of pressure (COP) changes greater than one standard error of measurement (SEM). Results With BBTW, people with MS had larger maximum changes in COP than healthy controls in the left-right direction but not in the anterior-posterior direction. COP changes greater than one SEM occurred in the same direction of weighting 20% of the time (95% confidence interval 5%, 34%), ranging from 10% to 28% across conditions and directions of postural sway. Direction of greatest weight placement did not match the direction of change in average COP in the majority of participants with multiple sclerosis or the healthy controls in EO or EC conditions (p<.0001). Conclusions If BBTW worked via a biomechanical shift of the center of mass, COP changes should match the direction of greatest weighting with BBTW. Our data allowed us to reject this hypothesis. Future research may explore alternative mechanisms of action underlying this intervention.