Objective: Physical activity confers numerous health benefits, yet few adults meet recommended physical activity guidelines. The impact of brief messages providing feedback on physical activity was tested in this study. Methods: Young adults were randomly assigned to one of three conditions: (1) feedback on how active others were, (2) feedback on how active others were plus a message of approval or disapproval depending on whether the participant was more or less active than average, or (3) no feedback (control condition). Participants used pedometers for eight weekdays and recorded their step counts each evening. The group receiving feedback on how active others were got information about the average number of steps taken by group members the previous day. The group that also received approval or disapproval received feedback about the group average, as well as a sad face if the participant was below the average or a happy face if the participant was above the average. The control group received no feedback throughout the study. Impacts of these feedback messages were compared on number of steps taken during the study.