Objective: M-CSF is a cytokine mediating the growth proliferation and differentiation of various cell types including macrophages trophoblast cells and osteoclasts. It plays an important role in immunity reproduction follicle development and ovulation. In the present study we describe the important role of the changes in serum M-CSF levels during the menstrual cycle in the process of follicular maturation ovulation implantation pregnancy and their response to ovarian stimulation with recombinant (r) FSH Materials and Methods: From an original sample of 95 IVF/ICSI patients (mean age = 33.8±5.4) serum and follicular fluid (FF) were collected on the day of follicular puncture (FP). The M-CSF levels were measured by ELISA technique. These patients were divided into two groups as follows: In Group 1 patients with the aetiology of tubal or male factor infertility were analysed for: a) correlation between serum and FF with respect to M-CSF and correlation between M-CSF and estradiol (E2) in serum; and b) comparison of M-CSF level in serum in response to ovarian stimulation and comparison of M-CSF level in serum between pregnant and non-pregnant patients. In Group 2: Patients (n=23) were monitored throughout the menstrual cycle until 4 weeks after embryo transfer. In this group M-CSF levels in serum were analysed throughout the different ovarian cycle phases and gestation. Results: M-CSF levels in FF were higher than in serum (p=0.01). M-CSF levels in serum increased from low through moderate to high response patients (p=0.001); pregnancy rates were 11.5% 22.5% and 51.7% respectively. M-CSF in serum increased throughout stimulation until the day of oocyte retrieval (p=0.006) and decreased until embryo transfer (ET p=0.03). In the post-retrieval days from the day of ET through implantation to the day of confirmation of pregnancy the M-CSF levels of those patients who became pregnant (n=13) increased significantly (p=0.03) and reached their highest level. After implantation the M-CSF levels decreased slightly and reached a plateau during gestation. Conclusion: our data show that M-CSF is involved in follicle development and ovulation and plays an important role in the maintenance of pregnancy. It could also be a predictor of embryo implantation for IVF outcome.