Epidermal development of human embryonic and fetal skin from the lower limb was studied using morphometric and statistical methods. Epidermal grooth, as defined by an increase in epidermal thickness and the number of cell layers, occurred in three distinct stages during the first and second trimesters. The first growth spurt occurred between 5 and 13 weeks estimated gestational age (EGA) and was followed by a plateau phase with little change in epidermal thickness from 14 to 21 weeks, after which the epidermis began to increase in height again. The periderm reached its maximal height by approximately 13 weeks EGA, and by 25 weeks was shed into the amniotic fluid. Thus, within a five-month period (5 to 25 weeks EGA) the epidermis changed from a single cell layer < 10 μ thick to a 10 to 12-cell layer, keratinized epithelium >60 μ thick. In contrast, epidermis from adult lower limb consisted of about 25 cell layers and was almost 75 μ in thickness. The age-related differences in epidermal thickness probably reflect changes in cell size and shape more than changes in the directional movement (apically vs. laterally) of proliferating keratinocytes, because the addition of cell layers throughout development was relatively constant. During the plateau phase, when there is a rapid increase in fetal growth rate, the suprabasal keratinocytes become more flattened, thereby allowing for the addition of new cell layers while maintaining a relatively constant epidermal thickness. The loss of glycogen reserves, along with other intrinsic cellular changes, probably contributes to the flattening effect. However, the internal expansion of growing tissues also may exert a mechanical pressure that could stretch the skin passively and influence epidermal structure.