Background: Breast cancer is the most prevalent cancer in our surgical practice in Egypt. Seroma is one of the most annoying complications after modified radical mastectomy and axillary clearance. Different modalities have been used to decrease the incidence of seroma. Objective: To assess the impact of using gelfoam in decreasing seroma formation following modified radical mastectomy. Patients and methods: This randomized controlled trial was done on eighty patients operated upon for modified radical mastectomy between January 2019 and November 2020 in Zagazig University Hospitals. Patients were randomly allocated into two groups; in each one forty patients. In the first group (group A) we used the gelfoam under the mastectomy and axillary flaps; in the second group (group B) we did the same surgical technique without using the gelfoam. Results: We noticed that the incidence of seroma formation among group (A) was 2.5% versus 17.5% in group (B), with statistically significant difference between both of them (P<0.05). Conclusion: The use of gelfoam is an effective procedure which reduces the incidence of seroma formation under mastectomy flaps.