Background: Skin tags (STs) are common benign skin tumors usually occurring on theneck and major flexors of elder people. STs are histologically composed of loosecollagen fibers and dilated capillaries enclosed in a hyperplastic epidermis. Objectives:We aimed at investigating the possible correlation between each of serum level ofprogesterone and estradiol with mast cells count (MCC) in STs among healthypregnant and non-pregnant females to check for their possible pathogenetic roles.Participants and Methods: Thirty participants with STs comprised two groups (15non-pregnant and 15 pregnant). One skin tag biopsy from each participant washistopathologically examined for mast cell count that was correlated with serumprogesterone and serum estradiol measured using specific ELISA kits. Results: Astatistically significant correlation between MCC and serum estradiol was detected inboth of the study groups. There was no statistically significant correlation betweenMCC and serum progesterone in both of the studied groups. MCC in STs wasstatistically higher in the pregnant than the non-pregnant group. Conclusion: Theremay be possible interaction of both mast cell and estradiol and both could be involvedin the pathogenesis of STs. Mohamed AA, El-Rifaie A-E, Gohary Y, El-Nesr KhA,Abd Eltawab A. Serum Progesterone and Estradiol Levels and the Mast CellsCount in Skin Tags among healthy Pregnant and Non-Pregnant Women:Correlation with Age of Pregnancy and Phases of the Menstrual Cycle. AUMJ,2015 September 1; 2(3): 47 - 56.