OBJECTIVE: To describe a case of cefoperazone-induced acute interstitial nephritis (AIN) in which the diagnosis was supported by renal biopsy. CASE SUMMARY: A 35-year-old woman presented to our hospital with decreased urine output and no past history of renal problems. Fifteen days prior to presentation, she had started treatment with intramuscular cefoperazone 1 g/day for a scalp infection. On day 12 of therapy, the patient noted decreased urine output, anorexia, and weakness, but she continued taking cefoperazone for 3 more days. Hemodialysis and oral prednisolone 1 mg/kg (30 mg twice daily) were started. Renal function returned to normal after 2 months of prednisolone treatment. DISCUSSION: Although AIN has been linked with other cephalosporins, as of June 18, 2004, to our knowledge, this is the first report of a cefoperazone-induced case. We based our diagnosis on the features of acute-onset renal failure, abnormal urinalysis findings, eosinophilia, inflammatory infiltrate in the interstitium, and recovery from renal failure after initiation of corticosteroid treatment. Application of the Naranjo probability scale indicated a probable relationship between the acute renal failure secondary to the possible AIN and cefoperazone therapy in this patient. CONCLUSIONS: This case indicates that cefoperazone, like other cephalosporins, can cause AIN. We recommend close monitoring of renal function in patients who are prescribed this drug.