OBJECTIVE To assess the effect of granulocyte macrophage colony-stimulating factor (GM-CSF) on unresponsive thin (<7 mm) endometrium in women undergoing frozen-thawed embryo transfer. METHODS A single-center, randomized, prospective study enrolled 304 women with thin unresponsive endometrium from Shanghai Ninth People's Hospital between March 2017 and May 2018. Of them, 161 patients received an intrauterine infusion of GM-CSF and 143 patients served as controls. After hysteroscopy, a gel-with or
without GM-CSF-was administered to fill the uterine cavity completely or up to 5 mL only. The primary outcome was confirmed pregnancy and secondary outcomes included endometrial thickness and implantation rate. RESULTS Patients who were administered GM-CSF had a significantly higher chemical pregnancy rate (35.3% vs 20.0%; P=0.009) and clinical pregnancy rate (28.6% vs 13.3%; P=0.005) compared with patients in the control group. Patients treated with GM-CSF had significantly higher endometrial thickness compared with controls (7.83+/-1.45 mm vs 7.37+/-0.70 mm, P=0.003). CONCLUSION GM-CSF therapy can effectively increase endometrial thickness and improve the clinical pregnancy rate in patients with persistent thin endometrium. The therapeutic role of GM-CSF for infertile women under in vitro fertilization and embryo transfer (IVF-ET) treatment can be further explored.