Abstract Background In the European Organization for Research and Treatment of Cancer (EORTC) randomized trial 30904, nephron-sparing surgery (NSS) was associated with reduced overall survival compared with radical nephrectomy (RN) over a median follow-up of 9.3 yr (hazard ratio: 1.50; 95% confidence interval CI, 1.03–2.16). Objective To examine the impact of NSS relative to RN on kidney function in EORTC 30904. Design, setting, and participants This phase 3 international randomized trial was conducted in patients with a small (≤5 cm) renal mass and normal contralateral kidney who were enrolled from March 1992 to January 2003. Intervention Patients were randomized to RN ( n = 273) or NSS ( n = 268). Outcome measurements and statistical analysis Follow-up estimated glomerular filtration rates (eGFR; milliliters per minute per 1.73 m2 ) were recorded for 259 subjects in the RN arm and 255 subjects in the NSS arm. Percentages of subjects developing at least moderate renal dysfunction (eGFR <60), advanced kidney disease (eGFR <30), or kidney failure (eGFR <15) were calculated for each treatment arm based on the lowest recorded follow-up eGFR (intent-to-treat analysis). Results and limitations With a median follow-up of 6.7 yr, eGFR <60 was reached by 85.7% with RN and 64.7% with NSS, with a difference of 21.0% (95% CI, 13.8–28.3); eGFR <30 was reached by 10.0% with RN and 6.3% with NSS, with a difference of 3.7% (95% CI, –1.0 to 8.5); and eGFR <15 was reached by 1.5% with RN and 1.6% with NSS, with a difference of –0.1% (95% CI, –2.2 to 2.1). Lack of longer follow-up for eGFR is a limitation of these analyses. Conclusions Compared with RN, NSS substantially reduced the incidence of at least moderate renal dysfunction (eGFR <60), although with available follow-up the incidence of advanced kidney disease (eGFR <30) was relatively similar in the two treatment arms, and the incidence of kidney failure (eGFR <15) was nearly identical. The beneficial impact of NSS on eGFR did not result in improved survival in this study population. Registration EORTC trial 30904; ClinicalTrials.gov identifier NCT00002473.