Most cases of acute hepatitis are caused by hepatitis viruses A, B or C. Diagnosis rests on the risk factor history and serological tests. In seronegative cases, consider other agents, such as Epstein-Barr virus and cytomegalovirus, drug reactions and autoimmune hepatitis. Hepatitis A and B can be prevented by appropriate use of highly effective, safe vaccines. Acute liver failure is an uncommon, devastating complication of acute viral hepatitis; continued vomiting, prolongation of prothrombin time and clouding of consciousness are indications for urgent transfer to a liver transplant unit. Hepatitis A is a simple, enterically transmitted illness that does not cause chronic hepatitis. 95% of adults recover from acute hepatitis B, whereas infection with hepatitis B virus acquired in childhood usually becomes chronic.