Few studies have been conducted documenting the length of time required for young children to achieve stable measures of running economy. Hence the purpose of this study was to quantify within- and between-day stability in running economy among young children. To address this issue, 30 6-year olds (16 girls, 14 boys) completed three test sessions within a 2 wk period. During Sessions 1 and 2 subjects performed three 5 min level treadmill runs at 2.23 m x sec(-1). During Session 2 expired air was collected during the last 2 min of each 5 min run (R1, R2, R3) and analyzed to determine VO2. In Session 3 subjects completed a single 5 min run (R4) at 2.23 m x sec(-1) and VO2 was measured during the last 2 min of running. Data analysis revealed no significant difference (p>0.05) in absolute or relative VO2 across trials. The average coefficient of variation for both absolute and relative VO2 among runs completed in Session 2 was 2.17 %, and the mean coefficient of variation for VO2 between R4 and the average VO2 of R1, R2, and R3 was 2.51 % and 2.35% for absolute and relative VO2, respectively. Moreover intraclass correlation coefficients for absolute and relative VO2 across all runs were 0.99 and 0.96, respectively. Viewed in concert, these results suggest that following 15 minutes of level treadmill running practice, stable within- and between-day measures of running economy can be obtained in young, prepubescent children.