The heat-stable antigen, CD24, is a cell-surface sialoglycoprotein expressed on immature cells that disappears after they have reached their final stage of differentiation. In mice, CD24 expression is preferentially upregulated in the developing mouse metanephros as compared with the surrounding intermediate mesoderm, but its role and expression in the developing human kidney has not been well described. Here we found in normal human fetal kidneys (8 to 38 weeks of gestation) that CD24 expression was upregulated and restricted to the early epithelial aggregates of the metanephric blastema and to the committed proliferating tubular epithelia of the S-shape bodies. Individual cells expressing CD24 were identified in the interstitium of later gestation and postnatal kidneys. In freshly isolated cells, FACS analysis identified distinct CD24(+) and CD24(+)133(+) cell populations constituting up to 16 and 14 percent, respectively, of the total cells analyzed. Early fetal urinary tract obstruction resulted in upregulation of CD24 expression both in developing epithelial structures of early stages and in the cells of the injured tubular epithelium of the later gestation kidneys. Our results highlight the cell-specific expression of CD24 in the developing human kidney and its dysregulation during fetal urinary tract obstruction.