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Medullary cystic kidney disease (MCKD) is usually associated with slowly progressive kidney injury. However, we encountered a case of MCKD with rapidly progressive kidney injury and irreversible renal dysfunction. A 63-year-old woman presented with a 4-month history of hypertension and rapidly progressive renal dysfunction. On admission, her blood pressure was slightly elevated (158/85 mmHg). The scrum creatinine (11.57 mg/dL) was markedly elevated. Urinalysis showed occult hematuria and proteinuria(1.06 g/gCr). /β2- microglobulin 45,000 μg/ L, N-acetyl-/β-D-glucosaminidase 5.6 U/L. Neither ultrasonography nor computed tomography revealed any evidence of renal medullary cysts. Both kidneys showed an irregular surface and enlargement. Microscopic evaluation of the renal biopsy revealed extensive tubular dilatation and atrophy with interstitial fibrosis. Often glomeruli, one had global sclerosis and the others were normal. The tubular dilatation was more marked in the distal than in the proximal tubules, according to the immunohistochemical findings of positivity for epithelial membrane antigen (EMA), a marker of distal tubules, and negativity for CD 10, a marker of proximal tubules. No immunoglobulin or complement deposition was detected in either the glomeruli or the tubules. Electron microscopy revealed disintegration of the tubular basement membrane with fragile thinning and lamination of the membrane. These pathological findings were compatible with MCKD. This was a case of MCKD diagnosed incidentally in an elderly patient who presented with rapidly progressive kidney injury accompanied by hypertension. Renal biopsy was necessary for the diagnosis.