The specific migration pathways which epithelial cells take as they migrate through the human villus is unknown, although there have been several speculations. The well-known phenomenon of 'gastric metaplasia' in the human duodenum is readily demonstrable by the diastase periodic acid Schiff (dPAS) method. However, there is no general agreement on whether the origin of this cell lineage is the villus epithelial cells or proliferative crypt cells. Using serial sections, model building and computer-aided three-dimensional reconstruction we have followed the pathway of migration of these metaplastic cells in human duodenal villi and report: (1) that these cells migrate in straight lines; (2) that migration is in relatively tight cohorts or migration streams; (3) that a single vestibule can supply cells to more than one villus; and (4) that the cell lineage has a complex origin, deriving either from Brunner's gland duct epithelium or from basal buds growing out of the crypts of Lieberkühn.