OBJECTIVE To assess the ability of granulocyte-macrophage colony-stimulating factor (GM-CSF) to regulate cyclooxygenase (COX) enzyme activity and prostaglandins (PGs) synthesis, specifically PGE2 production in stromal cells, neither of which have been addressed in the literature. METHODS Prospective study. METHODS Department of obstetrics and gynecology at a university hospital. METHODS Human luteal phase endometrium was obtained from surgical specimens (n = 6) for clinical indications. METHODS Confluent stromal cells were stimulated with GM-CSF. METHODS Expression of COX mRNA, COX enzyme activity, and PGE2 level in cultured stromal cells. RESULTS Confluent stromal cell cultures treated with P and E2 for 9 days were stimulated with GM-CSF. After treatment of 12 hours, low-dose GM-CSF (0.001-0.1 ng/mL) increased COX-2 mRNA levels in stromal cell, whereas high dose GM-CSF (1-100 ng/mL) decreased COX-1 and COX-2 mRNA levels. After treatment of 48 hours, low concentrations of GM-CSF (0.001-0.1 ng/mL) increased total COX and COX-2 enzyme activity, whereas high concentrations of GM-CSF (1-100 ng/mL) inhibited COX and COX-2 activity; The PGE2 levels decreased by 31% to 393.3 pg/mL (P < .05) with concentrations of GM-CSF increasing from 1 ng/mL to 100 ng/mL. CONCLUSIONS There appeared to be a biphasic pattern of COX-2 enzyme response to GM-CSF with low concentrations increasing activity and high concentrations inhibiting activity. It is possible that GM-CSF may provide critical regulation of PG production in the preimplantation period.