The surfaces and the sulci of the lateral hemispheres of 30 postmortem brain specimens were three-dimensionally measured. The coordinates of all the structures registered constitute a digital brain model. The models were scaled linearly until they fitted a defined cuboid. Then all the models within the reference volume, each represented by a set of points, were considered together. The reference volume was divided into small cubic volume elements (voxels). Two parameters of the voxels were studied: the point density of each structure per voxel and the contribution index which designates how many of the hemispheres contribute points to a certain voxel. The latter parameter may serve as an index for the presence probability of a sulcus, which means the probability with which a sulcus will be contained in a defined volume element. The presence probability of a sulcus corresponds well with its first appearance in embryological development. Spatial variation in the early developing principal sulci is far less than in those which develop in later stages. As regards some defined regions of the brain, statements about the probable location of cortical areas in relation to the commissural system can be made.