An 81-year-old woman was admitted to our hospital because of acute exacerbation of chronic renal failure. Her 24-h urine protein value was 0.37 g, but neither hematuria nor leukocyturia was seen. Renal biopsy specimens showed noncaseating granulomas with giant cells in the interstitium. A clinical examination revealed no evidence of tuberculosis, fungus, or malignancy. All of the drugs she had been taking were discontinued, but her renal function continued to deteriorate. No uveitis, erythema nodosum, or common macular skin lesion was seen. A computed tomography scan of the thorax and a total-body gallium-67 scan showed no abnormalities. The serum lysozyme level was greater than four times above normal. Finally, a diagnosis was made, of granulomatous interstitial nephritis due to isolated renal sarcoidosis. Treatment was started with 60 mg/day of prednisolone, and she had an excellent response. Her serum creatinine level decreased to the level shown before the acute exacerbation. It is important to consider renal sarcoidosis as a differential diagnosis in patients with severely progressive renal failure, because corticosteroid therapy is very effective.