Purpose: Modification of Diet in Renal Disease’ (MDRD) and 'Chronic Kidney Disease Epidemiology Collaboration’ (CKD-EPI) formulas are generally accepted and widely utilized tools to assess renal function. In this study, we aimed to investigate the power of the MDRD and CKD-EPI formulas, which are the two most used formulas in the measurement of eGFR in patients with multiple myeloma (MM). Materials and Methods: A total of 40 patients, newly diagnosis with MM, were consecutively enrolled to the study and their records were analyzed in terms of demographic features and laboratory parameters of the patients, type of paraproteinemia and stage of disease. MDRD and CKD-EPI formulas were used to evaluate renal function. Results: The difference found between basal and third-month estimated Glomerular Filtration Rate (eGFR) using MDRD (61±15.4 mL/min/1.73 m², third month 75±18.2 mL/min/1.73 m²) and CKD-EPI formulas (62±15.7 mL/min/1.73 m², third month 76±18.4 mL/min/1.73 m²) were significant. There was no significant difference between two formulas regarding basal and third month eGFR mean values. According to CKD staging, 12.5% of all subjects changed and 80% of them moved to better stage. Conclusion: CKD-EPI tends to produce higher eGFR values with compared to MDRD, especially when they are used to evaluate mildly impaired renal function. Hence, prevalence of renal disease was determined lower when assessed with CKD-EPI. Therefore, we recommend that it is important to use the same eGFR estimation formula for a consistent outcomes analysis.