Ranitidine is an H2 blocking agent widely used in the treatment of peptic ulcer disease. Although acute interstitial nephritis has been described in association with a related drug, cimetidine, this complication secondary to ranitidine has been reported only once before. We describe a patient who presented with acute renal failure and features of Fanconi syndrome in association with the use of ranitidine. Renal biopsy showed acute interstitial nephritis. The patient had renal tubular acidosis with alkaline urine, aminoaciduria, renal glycosuria, uricosuria, and phosphaturia. There was a marked disparity between the degree of elevation of urea nitrogen and creatinine concentrations in the serum that was attributable to diminished urea generation, since simultaneous measurement of inulin, creatinine, and urea clearances showed that they were comparably reduced. Urinary nitrogen loss in the form of aminoaciduria contributed significantly to decreased urea generation. Both acute renal failure and Fanconi syndrome resolved after treatment with prednisone.