The aim of this study is to share real-life data on the increase in creatinine due to CDK 4/6 inhibitor treatment and patients diagnosed with HR+/HER2-MBC and treated with ribociclib or palbociclib combined with ET were included in the study. While creatinine increase was observed in 17.9% (n = 19) of the 106 patients in the study population, 8.5% (n = 9) had Grade 1, 8.5% (n = 8) had Grade 2, and % 0.9 (n = 1) had Grade 3 creatinine elevation. The increase in creatinine occurred in 25% (n = 12) of ribociclib users and 12.1% (n = 7) of palbociclib users. No patient required a dose reduction or discontinuation of treatment due to elevated creatinine. Of the patients with high creatinine levels, 36.8% (n = 7) were over 65 years of age. Those with multiple comorbidities, blood urea nitrogen (BUN) >13.5 mg/dl, creatinine >0.66 mg/dl, BUN/creatinine ratio >19.95, glomerular filtration rate (GFR) >96.05 ml/min, and uric acid >4.69mg/dl. It was observed that the increase in the creatinine level was statistically significant (p <0.001). In conclusion, this study revealed that the increase in the serum creatinine secondary to ribociclib and palbociclib treatments is associated with kidney function tests and the number of concomitant diseases. Key Words: CDK 4/6 inhibitor, Creatinine elevation, Palbociclib, Ribociclib.