Background: Current investigation of treatment outcomes by clinicians is currently hampered by a lack of clinically viable tools. The use of activity monitors specifically validated for a population could help resolve this situation. Objectives: The purpose of the study was to validate an activity monitor for monitoring children with cerebral palsy. The study was designed to validate the duration of time spent sitting, standing and walking and the number of steps taken when being measured by the activPAL activity monitor. Study design: A validation study was undertaken. Methods: Observations of participants were carried out while completing a specifically designed activity course using video footage, which were then compared to the output from the activity monitor. Results: The activity monitor was found to be valid for the time spent standing and walking, the number of steps taken and the number of transitions. Conclusions: The results demonstrated that the monitor may prove useful to clinicians as a measurement outcome device for children with hemiplegic gait patterns resultant from cerebral palsy. However, the sensitivity of the device is variable, and further investigations are necessary to confirm it would also be able to detect minor changes after interventions.