Background: Multiple Myeloma Oncogene 1 (MUM1)/ interferon regulatory factor 4 (IRF4) is a member of the interferon regulatory factor family of transcription factors. It was thought to play an important role in the pre-B cell development, receptor editing, germinal center reaction as well as plasma cell generation. This retrospective study aimed to assess MUM1/ IRF4 diagnostic value as a step for further studies to determine its significance in therapeutic strategies for B cell malignancies. Methods: A total of 60 individuals were enrolled in this study. They were divided into two groups; the patient group, which included 40 newly diagnosed B cell malignancies patients (40 to 80 years old), 24 (60%) males and 16 (40%) females. They included 20 cases of Multiple Myeloma, 10 cases of Chronic lymphocytic Leukemia and 10 cases of Non-Hodgkin Lymphoma. The control group included 20 random subjects of cases other than B cell malignancies. Patients were diagnosed by flow cytometric immunophenotyping using routine panel of monoclonal antibodies for B cell malignancies followed by MUM1/IRF4 detection using anti MUM1/IRF4 monoclonal antibodies Kit (MACS, Milteny Biotec Inc., USA). Results: MUM1/IRF4 showed highest statistical difference among multiple myeloma group than among Chronic Lymphocytic Leukemia group and least among Non-Hodgkin Lymphoma and control groups. Average expressions were 93.1%, 24.2%, 6.5% and 0.6% respectively. Conclusion: MUM1/IRF4 had a significant role as a diagnostic marker in multiple myeloma.