Within- and between-day stability in locomotor energy use was quantified in 13 children with hemiplegic CP. During testing, subjects were familiarized with the laboratory environment (Session 1), performed three 5-min level treadmill walks (Trials 1, 2, and 3) at 0.67 m x s-1 (Session 2), and completed a single 5-min walk (Trial 4) at 0.67 m x s-1 (Session 3). In Sessions 2 and 3, HR was assessed and expired air was collected and analyzed to determine VO2. Data analyses revealed no significant difference (P > 0.05) in either net VO2 (ml x kg-1 x min-1) or EEIHR (b x m-1) across the three trials performed in Session 2 and between average measures of net VO2 and EEIHR quantified in Session 2 and those obtained in Session 3. Mean within-day coefficient of variation (CV) values for net VO2 and EEIHR were 8.6% +/- 8.5% and 13.9% +/- 7.8%, respectively. Analysis of between-day variability and energy expenditure revealed a between-day CV value of 13.1% for net VO2 and 24.0% for EEIHR. In addition, significant inverse relationships between GMFM scores and within-day (r = -0.61) and between-day (r = -0.58) CV values for net VO2 were detected. Viewed in concert, these data suggest that fairly stable within- and between-day measures of locomotor energy expenditure during level treadmill walking can be achieved in subjects with hemiplegic CP if testing is preceded by a short period of treadmill accommodation. However, children with greater motor dysfunction may require an extended period of treadmill accommodation to reduce trial-to-trial variability in walking energy use.