Occult hepatitis B is a recently recognized disease entity and is defined as the presence of HBV DNA in the serum and/or the liver tissue in absence of HBsAg. Occurrence of this HBV occult infection in chronic HCV patients is associated with a variety of disease entities including flares of liver enzymes, advanced hepatic fibrosis towards cirrhosis, reduced response rates to standard interferon and increased risk of hepatocellular carcinoma. Our study showed that occult hepatitis B occurs in 3.9% of chronic HCV patients under pegylated interferon/ribavirin combination therapy. Furthermore, this occult infection is not a statistically significant cause of therapeutic failure in chronic HCV patients.