We studied sexually dimorphic differences in the ilium using geometric morphometric analysis of 10 osteometric landmarks recorded by multislice computed tomography, based on three-dimensional reconstructions of 188 children (95 boys, 93 girls) of mixed origins living in the area of Toulouse, southern France, and ranging in age from 1 to 18 years. We used geometric morphometrics methodology first to test sexual dimorphism in size (centroid size) and shape (Procrustes residuals) and second to examine patterns of shape change with age (development) and size change with age (growth). On the basis of statistical significance testing, the ilium shape became sexually dimorphic at 11 years of age, although visible shape differences were observed as early as 1 year of age. There was no statistically significant difference in size between sexes. Trajectories of shape (development) and size (growth) differed throughout ontogeny and between sexes.