To assess the effect of granulocyte macrophage colony-stimulating factor (GM-CSF) on unresponsive thin (<7mm) endometrium in women undergoing frozen-thawed embryo transfer. 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 5mL only. The primary outcome was confirmed pregnancy and secondary outcomes included endometrial thickness and implantation rate. 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±0.70mm, P=0.003). 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. ChiCTR-IPR-17011242.