AbstractOccult HBV infection is a well‐recognised clinical entity characterised by the detection of HBV‐DNA in serum and/or in liver in the absence of detectable hepatitis B surface antigen (HBsAg). Occult HBV infection has been described not only in patients who have resolved an acute or chronic HBV infection but also in patients without any serological markers of a past HBV infection. Occult HBV infection in patients with chronic HCV infection may induce more severe liver disease and lower response rate to interferon treatment. The existence of occult HCV infections has been also reported more recently. Occult HCV infection is characterised by the presence of HCV‐RNA in liver and peripheral blood mononuclear cells in the absence of detectable serum HCV‐RNA. Occult HCV infection may occur under two different clinical situations: in hepatitis C antibody‐(anti‐HCV) negative and serum HCV‐RNA‐negative patients with abnormal liver function tests and in anti‐HCV‐positive patients who have no detectable serum HCV‐RNA and who have normal liver enzymes. The clinical relevance of occult HCV infections is still under investigation. Copyright © 2008 John Wiley & Sons, Ltd.