Recently, it has been shown that bone marrow (BM)-derived hematopoietic cells have critical roles in the tumor microenvironment as a major components of tumor stroma and regulate tumor progression. In addition, hematopoietic cells need chemokine signaling for their recruitment. On the other hand, COX-2 and endogenous prostaglandins are important determinants for tumor growth and tumor-associated angiogenesis. However, their precise mechanisms in stromal formation and angiogenesis remain elusive. Our recent data suggest that COX-2 inhibition reduced CXCL12/CXCR4 expression as well as tumor stromal formation, tumor-associated angiogenesis and tumor growth. Consistently, PGE₂ enhanced stromal formation, angiogenesis and CXCL12/CXCR4 expression. Moreover, a COX-2 inhibitor suppressed expression of a fibroblast marker (S100A4) in tumor stroma. These suppressive activities were found by either EP3 or EP4 knockout among 4 PGE2 receptors. Experiments using GFP-bone marrow chimeric mice revealed that CXCL12⁺CXCR4⁺S100A4⁺ fibroblasts dominantly composed stromal cells and most of which were recruited from BM. Additionally, fibroblasts were stimulated to produce CXCL12 by either EP3 or EP4 specific agonist in vitro. Therefore, COX-2/PGE₂-EP3/EP4 signaling may play a crucial role in tumor stromal formation and angiogenesis via CXCL12/CXCR4 chemokine system. These results may lead to new approaches in further studies and cancer treatment.