This study examined the emergence and characteristics of bilateral asymmetry of the upper limb during development in a medieval agricultural population from Hungary, and investigated the agricultural activity-types in a bioarchaeological and biomechanical context. The skeletal remains of 169 nonadult individuals were selected from the cemetery of Bátmonostor-Pusztafalu, providing 134 cases for humeral diameter and length, 70 cases for radial length and 62 cases for ulnar length measurements. Biological age was estimated by using tooth eruption. Age groups were defined on the basis of the development of motor skills in children. Statistical analyses included correlation, Chi-square test, Fisher analysis and ANOVA. Our investigation revealed that the frequency of asymmetry increased significantly during growth with a shift to the right side in all measurements and reached adult-like distribution in early childhood. The bilateral asymmetry increased with age in humeral length, while other measurements revealed no change. The magnitude of asymmetry decreased with age in humeral diameter, but remained constant in other measurements. Our observations strengthen the hypothesis that right-sided asymmetry develops gradually during growth. Our findings also illustrate the effects of medieval agricultural labour on upper limb asymmetry: mechanical loading has various effects during development both on the magnitude and on direction of asymmetry.