Purpose: To determine if the treatment with uterine infusion of granulocyte colony-stimulating factor (G-CSF) can improve endometrial thickness in an infertile woman with a double uterus, who consistently showed a thin endometrium in the late proliferative phase either in controlled ovarian hyperstimulation (COH) IVF-ET cycles or with graduated estrogen/sildenafil protocols for frozen embryo transfer (ET). Materials and Methods: A single uterine infusion of G-CSF was performed in the late proliferative phase in a woman who only attained a five-mm thickness despite a high dose vaginal and oral estradiol regimen plus sildenafil. Results: No increase was found within a couple days. Conclusions: A previous four-case study in another center found 100% improvement in the endometrial thickness in women with consistently thin endometria. Perhaps the uterine anomaly in the present case prevented the response of the endometrium.