The purpose of the study is to determine how effective preemptive tenofovir therapy is in preventing the re-activation of Hepatitis B infection, in patients who are receiving rituximab-based chemotherapy for Non-Hodgkin's Lymphoma or CLL/SLL. The rate of re-activation will be compared between patients who receive preemptive tenofovir and patients who receive tenofovir as needed.

Inclusion Criteria:

1. ≥ 18 years of age
2. Diagnosis of non-Hodgkin's lymphoma to be treated with rituximab-based chemotherapy
3. HBsAg negative, anti-HBc positive

Exclusion Criteria:

1. Current therapy with known activity against HBV
2. Screening ALT > 10 x ULN
3. Screening ALT >2 and <10 xULN with HBV DNA > 2000 IU/mL (indicates active HBV infection despite HBsAg negative and require antiviral therapy)
4. Life expectancy < 3 months
5. HBsAg positive
6. HIV co-infection
7. Active HCV co-infection (HCV RNA positive)
8. Creatinine clearance <50 mL/min
9. Intolerance to tenofovir
10. Women of child-bearing potential unwilling to take contraception during the study period