This study examined the validity of the Caltrac motion sensor for measuring physical activity in young children and described change over time in physical activity among 47 preschool children from obese and nonobese families. Children were videotaped in a controlled setting while wearing Caltrac motion sensors and also wore Caltracs at home. Caltrac readings and an observational measure of physical activity were significantly related in both Year 1 (r= .86) and Year 2 (r= .83), and outside play was associated with higher Caltrac scores (r=.43). In addition, physical activity among young girls with obese parents decreased significantly from Year 1 to Year 2.