Occult hepatitis B is defined as presence of HBV DNA in tissue or serum without hepatitis B surface antigen. The aim of this study is to determine frequency of occult hepatitis B among hepatitis C patients in Tehran and compare the route of transmission and liver enzymes between positive and negative HBV DNA patients.In a cross sectional study, serum of 103 hepatitis C cases (79.6% men and 20.4% women) were analyzed for s, x and core genes via a nested polymerase chain reaction technique.HBV DNA was detectable in serum of 20 patients (19.4%). No significant difference in age, sex and route of transmission were seen in HBV DNA positive and negative patients. In HBV DNA positive and negative groups, mean of AST was 73, 47 (p < 0.05) and mean of ALT was 76 and 36 respectively (p < 0.05).Occult hepatitis B was observed in a considerable number of hepatitis C patients in Tehran. It was associated with elevation in liver enzyme but was not related to route of transmission.