McSweeney M 1 , Wood M 1 , Grunewald S 1 , Cleary MA 1 , Abulhoul LH 1 1 Great Ormond St. Hospital for Children, London, United Kingdom Background: GSD III has both hepatic and muscle manifestations of varying degrees. Muscle disease affecting function is not always clinically apparent early in childhood and may go undetected. One aspect of the musculoskeletal system that lends itself to monitoring is gait. Importantly, objective assessment of gait has the potential to quantify clinical change. Method: Four children with a diagnosis of GSD III were assessed using the GaitRite system. Specific parameters of gait-velocity, cadence, step length and base of support were measured. Changes for each individual were analysed and compared with age-matched controls. Results: All patients had slower and less efficient gait patterns than their peers. The most notable differences were in velocity and asymmetry in step length and base of support indicating an unstable and inefficient gait. These differences were not apparent on routine clinical examination. Creatinine kinase levels were elevated in all patients. Conclusion: The GaitRite identified abnormalities in gait parameters in GSD III, highlighting alteration in musculoskeletal function prior to overt clinical signs. This reliable instrument can monitor the muscle disease in this rare disorder. It could have considerable potential in clinical management and as an outcome tool in evaluating potential therapies.