Variation in juvenile and adult humeral morphology between the two subfamilies of Hadrosauridae (Ornithopoda, Dinosauria) was examined. The sample consisted of five hadrosaurine and four lambeosaurine genera and included humeri from 28 to 65 centimeters in length. Nineteen landmarks associated with articular surfaces and muscle attachments were digitized from photographs of humeri in posterolateral view, and the data were analyzed using three morphometric techniques: Bookstein’s coordinates, Resistant-Fit Theta-Rho-Analysis, and Euclidean Distance Matrix Analysis. Differences in humeral shape were greater between adults of the two subfamilies than between the juveniles, and the differences between adults and juveniles were larger in lambeosaurines than in hadrosaurines. Adult lambeosaurines exhibited lateral and distal enlargement of the distal part of the deltopectoral crest and relative shortening of the shaft distal to the deltopectoral crest, resulting in the increased performance of the shoulder muscles, particularly Mm. pectoralis and deltoides clavicularis. The morphological change of the humerus during growth seems to be closely associated with the reduction of the humeral length relative to antebrachium. The morphological variations in hadrosaurid humeri relate to structural adaptations to bear increased body size to some extent but also to a behavioral specialization among some genera during growth.