Acute hepatitis B virus (HBV) infection is typically distinguished from chronic disease by a positive IgM anti-hepatitis B core antigen (anti-HBc) test. Patients with chronic hepatitis B remain hepatitis B surface antigen (HBsAg) positive, often with raised serum alanine aminotransferase (ALT) activities, for more than six months. The presence of hepatitis B e antigen (HBeAg) and HBV-DNA correlates with infectivity (although patients infected with the pre-core mutated virus may be HBeAg negative). Immunity after HBV infection is characterised by the presence of anti-HBs and anti-HBc antibodies. Patients who respond to interferon alfa treatment lose HBV-DNA and HBeAg from serum and their ALT values return to normal; some also lose HBsAg and acquire anti-HBs. Diagnosis of acute hepatitis C virus (HCV) infection remains largely dependent on history and exclusion, as anti-HCV antibodies may appear late or never at all, although HCV-RNA may be detectable on polymerase chain reaction (PCR) within days of infection. Second generation ELISAs detect a range of anti-HCV antibodies in chronic infections, and confirmatory RIBAs have reduced the incidence of false-positive results. Direct tests for HCV antigens in serum are not yet available, although PCR testing for HCV-RNA can be used to confirm viraemia. Patients who respond to interferon alfa treatment show continuous normalisation of serum ALT values, and some lose HCV-RNA. Relapse occurs in about half of all those who respond.