A common assertion in the literature is that Langhans cells in placental villi decline in number during gestation but this is a misinterpretation which may be caused by the greater growth of villous surface area compared with trophoblast volume. To test this possibility, human placentae were collected at 12-41 weeks of gestation for a cross-sectional study on the packing density of nuclei within villous trophoblast. Numbers of nuclei in the cyto- and syncytiotrophoblast were estimated using a design-based stereological device, the physical disector (parallel pairs of sections). Surface areas were estimated in order to assess the overall growth of villous arborizations. Packing densities of nuclei were calculated and expressed as numbers/1000 microns 2 of villous surface. Densities decreased during gestation and this can be explained by expansion of villous surface area and thinning of trophoblast. The biggest drop in packing density of cytotrophoblast nuclei (30 per cent) occurred between 17-21 and 22-26 weeks and this period coincided with the largest changes in villous surface area (62 per cent increase) and trophoblast thickness (30 per cent decrease). Results are consistent with the notion of an epithelial proliferative unit of constant volume and comprising about nine syncytiotrophoblast nuclei per Langhans cell.