BackgroundValidated, clinically meaningful outcome measures should be used to detect clinically relevant effects of treatments. Since the clinical heterogeneity in mitochondrial disorders is extremely wide, the selection and validation of outcome measures is challenging. Espe-cially for children, whom are developing and growing and even have a larger phenotypic heterogeneity compared to adults, this challenge has so far resulted in a lack of val-idated outcome measures. Gait analysis is an emerging method to quantify subtle changes in walking patterns of adults with neurological disorders and can provide in-sight in the effects of a therapeutic intervention. Based on the results of a validation study in m.3243A>G carriers, we included gait quantification as the primary outcome measure for the adult randomised, placebo-controlled, cross-over, phase 2 trial performed in this population in our centre (the KHENERGY trial). We hypothesise that gait analysis is also a feasible and reliable outcome measure for intervention studies in ambulatory children with mi-tochondrial disease.MethodsThe aim of this study was to select the opti-mal protocol to quantify gait patterns with the Gaitrite in paediatric mitochondrial patients, comparing a normal walking protocol and a post-exercise protocol. Ambula-tory children with a genetically confirmed mitochondrial disease are asked to walk across the Gaitrite three times for each trial and two times for each condition to estimate test-retest variability. First, the normal walking condition is tested. Subsequently, a 3-minte walking test is performed, followed by a post-exercise protocol. After 10 min of rest, a recovery condition is tested. Secondly, the gait patterns of the mitochondrial patients are compared to 5 age-and gender matched healthy controls to gain more insight in which walking parameters were affected by mi-tochondrial disorders.Results and conclusionThe results of this validation study will be presented.