Infants with Down syndrome (DS) are described as being less active and they also experience significant delays in motor development. It is hypothesized that early infant physical activity may be influential for the acquisition of independent walking. Physical activity was monitored longitudinally in 30 infants with DS starting at an average age of 10 months participating in a treadmill training intervention. Actiwatches were placed on infants' trunk and right ankle for a 24-hr period, every other month until walking onset. Data were analyzed to separate sedentary-to-light activity (low-act) and moderate-to-vigorous activity (high-act). Results showed that more leg high-act at an average age of 12 and 14 months is related to earlier onset of walking. It is recommended that early leg activity should be promoted in infants with DS. (Contains 1 table and 3 figures.)