A case of cefazolin-induced hypoprothrombinemia in a patient with renal failure is reported.A 50-year-old African-American woman was transferred from the orthopedics service to the internal medicine service for management of acute renal failure. Before her transfer, she had spinal surgery and subsequently developed a wound infection complicated by Escherichia coli bacteremia. After trials of multiple antibiotics, she developed acute interstitial nephritis and renal failure. On the day of her transfer to the internal medicine service, i.v. cefazolin sodium 1 g was administered every 24 hours to eradicate the E. coli detected in blood cultures. Her baseline International Normalized Ratio (INR) was 1.3. On day 7 of cefazolin therapy, her INR increased to 4.0. Because of her recent history of bleeding and hypotension, vitamin K 10 mg i.v. was administered, followed by 5 mg orally for the next two days. Her INR decreased and normalized at 1.1. The patient had no changes to other drug therapies and had no medical conditions known to independently affect prothrombin time during this episode. The score on the Naranjo et al. adverse-event probability scale revealed a probable relationship between cefazolin and hypoprothrombinemia in this patient.A patient with a postsurgery wound infection and acute renal failure developed hypoprothrombinemia after receiving cefazolin for seven days.