A 24-year-old woman with a history of penicillin allergy developed reversible acute renal failure after receiving cephalexin for 4 days. The patient experienced nausea, vomiting, diarrhea, pruritus, cough, and an elevated creatinine level of 2.2 mg/dl. The patient's creatinine level continued to rise, peaking at 5.3 mg/dl on hospital day 3. Nephrotoxic acute tubular necrosis was confirmed by electron microscopy. Within 1 month of discharge from the hospital, the patient's creatinine level decreased to 0.6 mg/dl. Although the renal injury most commonly associated with the cephalosporin class of antibiotics is allergic interstitial nephritis, currently available cephalosporins infrequently can cause direct tubular toxicity.