To evaluate the validity of the ActiGraph accelerometer for the measurement of physical activity intensity in children and adolescents with cerebral palsy (CP) using oxygen uptake (VO(2)) as the criterion measure. Thirty children and adolescents with CP (mean age 12.6 ± 2.0 years) wore an ActiGraph 7164 and a Cosmed K4b(2) portable indirect calorimeter during four activities; quiet sitting, comfortable paced walking, brisk paced walking and fast paced walking. VO(2) was converted to METs and activity energy expenditure and classified as sedentary, light or moderate-to-vigorous intensity according to the conventions for children. Mean ActiGraph counts min(-1) were classified as sedentary, light or moderate-to-vigorous (MVPA) intensity using four different sets of cut-points. VO(2) and counts min(-1) increased significantly with increases in walking speed (P < 0.001). Receiver operating characteristic (ROC) curve analysis indicated that, of the four sets of cut-points evaluated, the Evenson et al. (J Sports Sci 26(14):1557-1565, 2008) cut-points had the highest classification accuracy for sedentary (92%) and MVPA (91%), as well as the second highest classification accuracy for light intensity physical activity (67%). A ROC curve analysis of data from our participants yielded a CP-specific cut-point for MVPA that was lower than the Evenson cut-point (2,012 vs. 2,296 counts min(-1)), however, the difference in classification accuracy was not statistically significant 94% (95% CI = 88.2-97.7%) vs. 91% (95% CI = 83.5-96.5%). In conclusion, among children and adolescents with CP, the ActiGraph is able to differentiate between different intensities of walking. The use of the Evenson cut-points will permit the estimation of time spent in MVPA and allows comparisons to be made between activity measured in typically developing adolescents and adolescents with CP.