We describe four patients who developed an encephalopathic syndrome characterized by obtundation or stupor, myoclonic jerks, and asterixis in association with cefuroxime therapy. Three patients had renal failure. These cases suggest that cefuroxime in overdose or in conventional doses in patients with renal failure can cause a reversible encephalopathy. This syndrome may have been unrecognized because it usually occurs in severely ill patients with additional causes for encephalopathy.