The incidence of hepatocellular carcinoma (HCC) in Mongolia is far higher than that of any other cancer in the country, and among the highest worldwide. The relative importance of infection with hepatitis B virus (HBV) and hepatitis C virus (HCV) is unclear. We reviewed (i) medical records for 963 patients with HCC and 941 patients with cirrhosis admitted for the first time to the National Cancer Center of Mongolia and the National Center for Communicable Diseases, respectively, from 2000 to 2009,and (ii) articles published from 1990 to 2010 on the seroprevalence of hepatitis B surface antigen (HBsAg) and antibodies against hepatitis C virus (anti-HCV) among individuals with and without liver disease. Among those with HCC, the seroprevalence of HBsAg, anti-HCV and dual infections was 50, 27 and 21%, respectively. Corresponding percentages among the patients with cirrhosis were 40, 39, and 20%. In both diseases, HCV infection was relatively more prevalent in women than in men and, in cirrhosis, inpatients older than 45 years of age. In healthy individuals,from published articles, anti-HCV seroprevalence steadily increased with age (from 3% at age 0-5 years to 34% at age ≥ 50 years), whereas HBsAg seroprevalence stayed constant at about 8%. The future benefit of childhood vaccination against HBV in Mongolia will be undermined by the consequences of a severe HCV epidemic and a uniquely high burden of dual infections.