Abstract The aim of this study was to establish the suitability of the North Star Ambulatory Assessment for use in young boys with Duchenne Muscular dystrophy We studied 147 typically developing and 144 boys affected by Duchenne Muscular dystrophy between the age of 3 and 5 years. More than 85% of the typically developing boys by the age of 4 years had full scores on all the items with total scores >33/34. Before the age of 4 years more than 15% of the typically developing boys did not achieve full scores on all the items. Some items, such as standing on one leg, showed significant improvement with age. In contrast, other activities were rarely achieved even in the older boys. Even if there was a progressive increase in scores with age, both total and individual item scores in Duchenne were still far from those obtained in the typically developing children of the same age. Our findings suggest that the North Star Ambulatory Assessment can be reliably used at least from the age of 4 years. Longitudinal natural history data studies are needed to assess possible changes over time and the possible effect of early steroids.