The hypothesis that patterns of sexual dimorphism in transverse dimensions of the metacarpal only reflect sex differences in body size was investigated in a sample of 324 Mexican school children 6.00–10.99 years of age. Several patterns of sexual dimorphism in second metacarpal dimensions are seen with regard to stature and weight. Sex differences in medullary diameter and percent cortical area are not related to sexual dimorphism in body size, while sex differences in cortical thickness are related to, but not fully explained by, differences in body size. Dimorphism in metacarpal diaphyseal diameter is related to differences in body size, but, at a constant body size and age, boys still have significantly larger diaphyseal diameters than girls. Finally, sexual dimorphism in cortical area is closely associated with sex differences in body siz