This study examined sources of variability in physical activity (PA) of youth with developmental disabilities (DD), and determined the optimal number of days required for monitoring PA. Sixteen youth with DD wore two pedometers and two accelerometers for 9 days, including 5 weekdays (W) and 2 weekends (WK). A two-facet in fully crossed two-way ANOVAs were employed to estimate sources of variability across W, WK, and W and WK combined (WWK) for each device. Primary sources of variability were the person and the person by day interaction for both devices. Using a pedometer, four, six, and eight days of measurements were required to determine typical PA levels of the participants during W, WK, and WWK, respectively. Using one accelerometer, four days of measurements were estimated across all days. (Contains 6 tables and 2 figures.)