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• Daily gait performance and physical activity are measurable with inertial sensors. • A 5-sensor setup reliably measures gait performance in children with cerebral palsy. • Better reliability was found when measuring 2 school days than 1 school and 1 weekend day. • Physical activity measures were reliable when comparing the same school day weeks apart. There is a lack of objective and reliable tools to measure walking performance in children with cerebral palsy (CP). To evaluate the reliability of inertial measurement units (IMUs) measuring daily life walking performance and physical activity (PA) in children with CP and healthy controls. Algorithms were developed to analyse data collected with IMUs during 2 standard school days of the same week and 1 weekend day in 15 children with CP and 14 controls. Additionally, within a clinical trial, 10 children with CP were measured twice, on the same weekday 2 to 4 weeks apart. Relative and absolute reliabilities of PA (% time walking, standing, sitting/lying) and gait parameters (e.g., velocity, cadence) were evaluated by using the intraclass correlation coefficient (ICC) and minimal detectable change (MDC 95), comparing 2 school days of the same week, a school day with a weekend day, and the same weekday 2 to 4 weeks apart. For the 15 children with CP (mean [SD] age 13.5 [3.4] years), ICCs were very high (0.70–0.98) when comparing gait parameters for 2 school days. ICCs were lower when comparing 2 school days for 14 control children (mean [SD] age 13.9 [3.0] years) and lowest when comparing a school day with a weekend day for both CP and control children. ICCs for PA were 0.90–0.91 when measuring the same weekday 2 to 4 weeks apart but were very low when comparing 2 school days of the same week or a school day with a weekend day. MDC 95 values were high for both groups and all comparisons but comparable with findings of in-lab studies of similar parameters. Our IMU and algorithm setup appears to be a reliable tool to measure daily life gait parameters in children with CP when repeatedly measured on 2 school days. PA was also reliably assessed but when measuring the same school day some weeks apart. However, the high MDC 95 values question whether the setup can be used as a responsive outcome measure of interventions. [ABSTRACT FROM AUTHOR]