Abstract Purpose: To compare the functional outcomes of patients who underwent partial (PN) or radical nephrectomy (RN) for clinical T1 (cT1) renal tumors using the Kidney Cancer Database of the Urooncology Association, Turkey. Methods: We retrospectively reviewed 1004 patients who underwent PN and RN for cT1 renal tumors at multiple academic tertiary centers between 2000 and 2018. Patients with preoperative end-stage chronic kidney disease and/or metastatic disease were excluded. Results: There were 452 patients in the PN group and 552 patients in the RN group. The eGFR was significantly reduced in both groups on postoperative day one (PN=13.7 vs. RN=19.1 ml/min/1.73 m2: p<0.001). In the PN group, eGFR showed a tendency to recover according to a quadratic pattern and reached preoperative levels in the first and third years (95.6±28.8 ml/min/1.73 m2 and 96.9±28.9 ml/min/1.73 m2, respectively), with no significant difference between the eGFRs in the 1st and 3rd years (p=0.710). To define groups at risk, different cut-off values for the GFR were considered. Among patients with a baseline GFR<90, the RN cohort had significantly lower eGFRs in the first and third years than the PN cohort (p=0.02). Logistic regression showed that comorbidities, coronary artery disease, diabetes and hypertension had no adverse impacts on the changes in the eGFR (p=0.60, p=0.13, and p=0.13, respectively). Conclusion: For the treatment of stage T1 kidney tumors, the first choice should be open or laparoscopic partial nephrectomy due to the superior long-term preservation of renal function and overall survival, regardless of age and comorbidities.