Urinary excretion of lipocalin-type PGD 2 synthase (L-PGDS), which converts PG H 2 to PGD 2, increases in early diabetic nephropathy. In addition, L-PGDS expression in the tubular epithelium increases in adriamycin-induced nephropathy, suggesting that locally produced L-PGDS may promote the development of CKD. In this study, we found that L-PGDS–derived PGD 2 contributes to the progression of renal fibrosis via CRTH2-mediated activation of Th2 lymphocytes. In a mouse model, the tubular epithelium synthesized L-PGDS de novo after unilateral ureteral obstruction (UUO). L-PGDS-knockout mice and CRTH2-knockout mice both exhibited less renal fibrosis, reduced infiltration of Th2 lymphocytes into the cortex, and decreased production of the Th2 cytokines IL-4 and IL-13. Furthermore, oral administration of a CRTH2 antagonist, beginning 3 days after UUO, suppressed the progression of renal fibrosis. Ablation of IL-4 and IL-13 also ameliorated renal fibrosis in the UUO kidney. Taken together, these data suggest that blocking the activation of CRTH2 by PGD 2 might be a strategy to slow the progression of renal fibrosis in CKD.