Our goal was to determine the differences in epidermal proliferation between keratinocytes obtained from young and old human skin. When the skin is wounded, it has to go through a rehabilitation process. The length of this process can vary in time depending on the health of the cells and their environment. With increased age, wound healing becomes slower. Earlier in vitro work has shown that young skin cells migrate faster in low oxygen tension (4%) than high oxygen tension (21%), whereas older skin cells migrate faster in higher oxygen tension than in low oxygen tension (Fig 2). Although this cell assay showed differences between young and old keratinocytes, it was unclear if the differences were due to an increase in cell migration, proliferation, or both. Thus, our aim was to resolve this question. Using the same scratch assay, we counted the number of metaphase cells in cultures of young and old human skin keratinocytes grown at different oxygen tensions. We found that scratching caused young keratinocytes to increase their proliferation, no matter in which oxygen tension they were grown, whereas old keratinocytes did not increase their proliferation. Thus, age appears to be the differentiating factor, rather than oxygen tension.