Prostaglandin E2, which is known to contribute to cancer progression, is inactivated by the catabolic enzyme, 15-hydroxyprostaglandin dehydrogenase (PGDH), which has tumor-suppressor activity in lung, colon, breast, and gastric cancers. Therefore, we evaluated the expression of PGDH in human bladder cancer tissue specimens and cell lines. Immunoperoxidase staining of bladder cancer tissues demonstrated that (1) PGDH is highly expressed by normal urothelial cells but (2) reduced in many low stage (Ta/Tis) bladder cancers, and (3) PGDH is completely lost in most invasive bladder cancers. Of eight cancer cell lines tested, only two relatively well-differentiated bladder cancer cell lines, RT4 and UM-UC9, expressed PGDH. Moreover, inhibition of PGDH expression in well-differentiated RT4 cells using small inhibitory RNA or short hairpin RNA resulted in a more aggressive phenotype with increased motility and anchorage-independent growth. Additionally, PGDH knockdown affected prostaglandin E2 signaling as measured by cAMP generation. These data indicate that loss of PGDH expression contributes to a more malignant bladder cancer phenotype and may be necessary for bladder cancer development and/or progression.