Low levels of physical activity and an associated rise in obesity prevalence in adults and children are causes for concern. One approach to increasing daily physical activity levels is through active commuting to work or school. The recent interest in promoting active commuting necessitates an accurate and feasible method to measure ambulatory physical activity and track routes, for sound evaluation of these interventions. Recent developments in physical activity monitoring have led to the use of global positioning systems (GPS), which captures movements in a way that other methods cannot. The Trackstick[TM] Super (Telespial Systems, Inc., Marina Del Rey, CA) GPS tracking device measures 11.43 cm x 3.18 cm x 1.91 cm and is powered by two AAA batteries. Recorded data include time, date, latitude, longitude, altitude, temperature, and direction of movement. Based on the manufacturer's specifications, the authors selected the Trackstick Super to evaluate a school-based active commuting intervention. The purpose of this study was to determine if the Trackstick Super GPS device performs to the manufacturer's advertised specifications under controlled conditions. Specifically, the authors wanted to establish whether the device (a) consistently records data every 5 s, (b) records locations with a horizontal accuracy of 2.5 m, and (c) accurately measures the distance of a known route. The authors conclude that the Trackstick Super GPS performs substantially below the manufacturer's stated performance specifications. During this study, the devices recorded less frequently than specified, and the average distance of recorded locations from known points was more than twice the advertised distance. Furthermore, the devices overestimated walking distance by approximately 11-21%, and 25% malfunctioned, causing considerable data loss. Despite these findings the Trackstick device may have merit in some research situations, depending on the main outcome measure (e.g., distance, speed/intensity, or location/route) and desired level of accuracy. (Contains 1 table and 4 figures.)