Purpose. To determine the degree and predictors of renal function loss of the operated kidney following nephron-sparing surgery (NSS). Material and methods. The study group included 113 patients with renal mass who underwent NSS at our institution. QDMSA before and 3–6 months after surgery was used for evaluation differences in renal function of each kidney. Mean change of percent uptake by the kidney was correlated with various clinical and pathological variables. Results. The overall average decrease of renal function of the operated kidney as measured by QDMSA was 10.5% ± 2.6 SER. Among the studied variables, the most important predictors of postoperative ipsilateral residual kidney function were estimated blood loss (EBL), 𝑃 = 0 . 0 0 0 3 , duration of warm ischemia, 𝑃 = 0 . 0 0 8 , patient’s age at surgery, 𝑃 = 0 . 0 2 4 , method used for tumor bed closure, 𝑃 = 0 . 0 6 , and location of the lesion, 𝑃 = 0 . 0 8 . Conclusions. Carful hemostasis, minimal duration of arterial clamping, and use of tissue adhesives to seal tumor bed are associated with maximal preservation of postoperative residual renal function after NSS. These variables should be considered by the operative team when planning the surgical procedure .