BACKGROUND The aim of this study was to assess long-term residual kidney function after unilateral nephrectomy for non-syndromic Wilms tumor (NSWT). METHODS Of the patients who underwent one-sided NSWT at Tohoku University Hospital between 1977 and 2003, nine were followed up until age ≥18 years. For these nine patients, we retrospectively evaluated estimated glomerular filtration rate (eGFR) in childhood (3-10 years old), adolescence (11-17 years old) and adulthood (≥18 years). RESULTS Mean age at the last follow up was 23.0 years. Tumor classification was as follows: stage I tumor, n = 6; stage II tumor, n = 3; mixed-type nephroblastoma, n = 8; and congenital mesoblastic nephroma, n = 1. Mean eGFR was 101.3 ± 21.2 mL/min/1.73 m2 in childhood, 106.0 ± 32.1 mL/min/1.73 m2 in adolescence and 100.5 ± 20.7 mL/min/1.73 m2 in adulthood. Therefore, no significant change in eGFR was observed over the three life stages evaluated. Further, none of the patients met the diagnostic criteria for chronic kidney disease by early adulthood. CONCLUSIONS eGFR after unilateral nephrectomy in patients with NSWT remained ≥60 mL/min/1.73 m2 during the transition from childhood to early adulthood, with no development of chronic kidney disease or end-stage kidney failure.