Mice lacking a functional cyclooxygenase-2 (COX-2) gene develop abnormal kidneys that contain hypoplastic glomeruli and reduced proximal tubular mass, and they often die of renal failure. A comparison of kidney-specific gene expression between wild-type and COX-2-deficient mice by cDNA microarrays revealed that although more than 500 mRNAs were differentially expressed between the two strains of mice depending on their ages, the genes encoding pre-pro-epidermal growth factor (pre-pro-EGF) and Tamm-Horsfall protein (THP)/uromodulin were aberrantly expressed in the kidneys of COX-2 −/− mice at all stages of their development. Downregulation of EGF could potentially affect renal development, and THP/uromodulin gene has been implicated in abnormal kidney development and end-stage renal failure in humans. We assessed in detail mechanism of defective THP/uromodulin gene expression and its potential consequences in COX-2-deficient mice. Consistent with the microarray data, the steady-state levels of THP/uromodulin mRNA were severely reduced in the COX-2 −/− kidney. Furthermore, reduced expression of renal THP/uromodulin, as assessed by Western blot and immunohistological methods, was closely corroborated by a corresponding decline in the urinary secretion of THP/uromodulin in COX-2 −/− mice. Finally, we demonstrate that the bladders of COX-2 −/− mice, in contrast to those of the wild-type mice, are highly susceptible to colonization by uropathogenic Escherichia coli.