Background & Objective:During the last decade, biological markers of breast cancer have been considered to predict the degree of histology, behavior, and extent of tumor invasion and the possibility of lymph node involvement. The aim of this study was to evaluate the expression of GCDFP-15 in different grades of invasive ductal carcinoma, as the most common type of breast cancer.Methods:In this retrospective study, paraffin blocks of tumors of 60 breast cancer patients registered in the histopathology laboratory of Imam Khomeini Hospital in Ahvaz between 2019 and 2020 were reviewed. Information on grade, invasion, stage and lymph node involvement was extracted from the pathology reports and immunohistochemical staining for GCDFP-15 was performed. Data were analyzed by SPSS 22.Results:GCDFP-15 marker expression was observed in 20 out of 60 breast cancer patients (33.3%). GCDFP-15 staining intensity was weak in 7 cases (35%), moderate in 8 cases (40%), and strong in 5 cases (25%). The patient's age and sex showed no significant relationship with the expression of GCDFP-15 and intensity of staining. Expression of the GCDFP-15 marker was correlated significantly with tumor grade, stage, and vascular invasion (P<0.05)) and its expression was higher in tumors with a lower grade, less depth of invasion, and no vascular invasion but unrelated to perineural invasion, lymph node involvement, and tumor size. The intensity of staining for GCDFP-15 showed significant relationship with the tumor grade (P<0.0001) but unrelated to the other factors.Conclusion:GCDFP-15 marker may be significantly associated with tumor grade, depth of invasion, and vascular invasion, thus can be used as a prognostic marker.