We describe here two sisters with nephronophthisis, which was not detected until the development of endstage renal failure. Twenty- and 15-year-old female siblings were admitted to our hospital for further examination of renal dysfunction. No urinalysis abnormalities had been found in yearly health checks in either patient. The serum creatinine level was 7.2 mg/dl in case 1 (the 20-year-old) and 6.4 mg/dl in case 2. Medical history, physical findings, and laboratory tests showed no evidence of urinary tract infection, use of any drugs, arthritis, or skin eruptions. To identify the cause of the renal failure, open left renal biopsies were performed in both patients. Histopathological findings were very similar in the two patients and included marked tubular and interstitial changes (tubular dilatation, focal tubular atrophy, interstitial fibrosis, and infiltration of mononuclear cells). The glomeruli were devoid of mesangial proliferation, mesangial expansion, and adhesion of Bowman's capsule. Based on the clinical and pathological findings, the final diagnosis was nephronophthisis in both patients. It is important to remember that some progressive renal diseases, including nephronophthisis, cannot be detected even by annual urinary screening tests, which are widely performed in Japan.