Antiviral resistance remains an important issue for long-term NA therapy. For lamivudine (LAM), the rtM204V/I and rtL180M mutations occur in more than 70% after 5 years of therapy. In Korea, primarily owing to limited subsidization policy in the health insurance system, many patients with LMV-resistance had been treated with either rescue ADV or ETV 1.0 mg monotherapy, ultimately leading to the higher prevalence of MDR strain. For those patients, rescue therapies of combining ADV with either ETV or LAM had been tried, but frequently with suboptimal responses. Rescue TDF monotherapy or TDF-based combination therapy are available in Korea for patients who had "difficult-to-treat" antiviral resistance owing to prior treatment failures. However, which is the better has not been evaluated yet. A long-term efficacy and safety of TDF-based rescue therapies in real practice for those patients should be necessary to revise the Korean guideline for the treatment of chronic hepatitis B in near future.

Inclusion Criteria:

• more than 20 years old adults
• chronic hepatitis B
• the patients treated by tenofovir alone or tenofovir based combination therapy because of the previous treatment failure
• the patients who agree and singed on the consent form

Exclusion Criteria:

• co-infected patients with HCV, HDV or HIV
• pregnancy or breast feeding woman or female patients who are planning to be pregnant
• past history with hepatocellular carcinoma
• combined with other liver disease including wilson, alcoholic, NASH, alpha-1 antitrypsin deficiency liver disease.
• patients with hypersensitivity for drugs
• patients who were enrolled in other clinical study within 60 days
• patients who were eligible for the clinical study according to the investor

Chronic hepatitis B patients who had "difficult-to-treat" antiviral resistance owing to prior treatment failures