Quantitative evaluation of function, in children with physical disabilities, has to date been mainly focused on laboratory-based measures. However, the measurement of activity in the community may have a more direct relationship with physical function, health, and well-being. We assessed the utility of a remote activity monitor, the Uptimer (National Aging Research Institute of Melbourne, Melbourne, Australia), to measure one aspect of physical function, time spent in the upright position, in a consecutive cohort of 300 children with cerebral palsy who attended an orthopedic outpatient department. The Uptimer was found to be a valid and reliable tool to measure the amount of time children spent on their feet each day. Uptime was closely related to the severity of cerebral palsy and had excellent correlations with validated instruments including the Pediatric Orthopaedic Data Collection Instrument (PODCI), the Child Health Questionnaire (CHQ), and the Functional Mobility Scale (FMS). Uptime complements any quantitative functional assessment of impairments in children with cerebral palsy.