The postnatal development of interhemispheric asymmetry was analyzed in the primary motor cortex (area 4) of 20 human brains with quantitative cytoarchitectonic techniques. The volume fraction of cortical tissue occupied by cell bodies (grey level index) was determined by automated image analysis. In children as well as in adults, the volume fraction of cell bodies averaged over all cortical layers was greater on the right than on the left. Thus, the space between cell bodies, i.e. the volume fraction of neuropil containing axons, dendrites and synapses, was greater in the left than in the right primary motor cortex. At the level of single layers, however, interhemispheric asymmetry of the neuropil volume fraction differed between age groups. The supragranular layers were significantly less asymmetrical in children than in adults, whereas the infragranular layers showed a similar degree of asymmetry in both age groups. Thus, the postnatal development of the architectonic asymmetry in the supra- and infragranular layers of area 4 follows the same sequence of maturation as found during neuronal migration, i.e. an inside-to-outside gradient. Comparing the layer-specific developmental pattern with available functional data, it was found that the structural maturation of interhemispheric asymmetry in the supragranular layers correlates with the development of hand preference.