CD24, or heat stable antigen, is a cell surface sialoglycoprotein expressed on immature cells that disappears after the cells have reached their final differentiation stage. CD24 may be important in human embryonic kidney epithelial cell differentiation. In mice, CD24 expression is up-regulated in the early metanephros and localized to developing epithelial structures but the role and expression of CD24 in the developing human kidney has not been well described. In normal human fetal kidneys from 8 to 38 weeks gestation, CD24 expression was up-regulated and restricted to the early epithelial aggregates of the metanephric blastema and to the committed proliferating tubular epithelia of the S-shape nephron; however individual CD24+ cells were identified in the interstitium of later gestation and postnatal kidneys. In freshly isolated cells, FACS analysis demonstrated distinct CD24+ and CD24+133+ cell populations, constituting up to 16% and 14% respectively of the total cells analyzed. Isolated and expanded CD24+ clones displayed features of an epithelial progenitor cell line. Early fetal urinary tract obstruction resulted in an upregulation of CD24 expression, both in developing epithelial structures of early gestation kidneys and in the cells of the injured tubular epithelium of the later gestation kidneys. These results highlight the cell specific expression of CD24 in the developing human kidney and dysregulation in fetal urinary tract obstruction.