Objective: In humans, the endometrium consists of stromal and epithelial cells. The interaction between stromal cells and epithelial cells is important in modulating the marked morphologic changes in endometrium during the menstrual cycle and in the preparation of a suitable environment for embryonic implantation and development. Granulocyte-macrophage colony-stimulating factor (GM-CSF) mRNA and protein are expressed in epithelial and stromal cells of human endometrium, but the epithelial cells are the major source of biologically active GM-CSF. In contrast, receptors for GM-CSF are found primarily expressed on stromal cells, suggesting that GM-CSF may be an autocrine/paracrine factor in human endometrium. Prostaglandins (PGs) are vasoactive substances in the endometrium involved in the control of cytokine release, cell growth, differentiation and blood flow. Two distinct cyclooxygenase (COX) isoenzymes have been described and designated as COX-1 and COX-2. GM-CSF has been showed to stimulate COX-2 expression in neutrophil cells. In the present study, we investigate the effects of GM-CSF on the production of PG in stromal cells and examine the ability of GM-CSF to regulate cyclooxygenase activity in human stromal cells.