Step-by-step variations in step width have been hypothesized to reflect adjustments to swing foot placement in response to preceding frontal plane trunk kinematics. The present study tested this hypothesis while 12 younger and 11 older subjects walked on treadmill for 10min at a self-selected velocity. The relationship between step-by-step variations in step width and frontal plane trunk COM kinematics was determined using multiple regression analysis. Trunk kinematics at midstance were significantly (p<0.001) and strongly (R2=0.54) related to the subsequent foot placement supporting the primary hypothesis. Additionally, this relationship was significantly affected by age (p<0.001) and stepping limb (p<0.001). These results implicate feedback driven control of foot trajectory during the swing phase. Further, they provide a biomechanical framework by which loss of frontal plane dynamic stability may result from a step width that is insufficient to decelerate and redirect trunk kinematics in preparation for the next step. © 2010 Elsevier B.V.