Background: This study examined the association between parent and child change in physical activity during a family-based intervention for child weight gain prevention. Methods: Daily step counts were recorded for parents and children in 83 families given a goal to increase activity by 2000 steps per day above baseline. Linear mixed effects models were used to predict child change in daily step counts from parental change in step counts. Results: Both maternal (P < .0001) and paternal (P < .0001) change in step counts for the current day strongly predicted child change in step counts for that day. On average, a child took an additional 2117.6 steps above baseline on days his or her mother met her goal versus 1175.2 additional steps when the mother did not meet her goal. The respective values were 1598.0 versus 1123.1 steps for fathers. Day of week moderated the maternal effect (P = .0019), with a larger impact on Saturday and Sunday compared with weekdays. A similar but nonsignificant pattern was observed for fathers. Conclusions: Encouraging parents to increase physical activity, particularly on weekends, may be a highly effective way to leverage parental involvement in interventions to increase children's physical activity. © 2012 Human Kinetics, Inc.