OBJECTIVE. To serially evaluate the viral kinetics of occult hepatitis B virus infection in lymphoma patients and perform a correlation with clinical outcomes. DESIGN. Case series with 1-year follow-up. SETTING. Regional hospital, Hong Kong. PATIENTS. Consecutive patients who were newly diagnosed to have lymphoma in the hospital between 1 April 2007 and 31 March 2008 were tested for hepatitis B (HB) surface (s) antigen (Ag), anti-HBs antibody (Ab) and anti-HB core (c) Ab. Seropositive occult hepatitis B patients as defined by being negative for HBsAg but positive anti-HBsAb and/or anti-HBcAb without a hepatitis B vaccination history were recruited. Serum HBsAg, anti-HBsAb, anti-HBcAb, hepatitis B virus deoxyribonucleic acid (DNA) level, and liver biochemistry were checked at baseline and every 4 weeks during and after chemotherapy until 12 months after the completion of chemotherapy or death. Entecavir was started if patients developed biochemical flare-up of hepatitis B associated with virological rebound. The prevalence and course of hepatitis B virus-related hepatitis, as well as any temporal relationship to viral kinetics and clinical hepatitis, were assessed. RESULTS. Of 47 patients tested, 10 (21%) with lymphoma were seropositive occult hepatitis carriers. Their median baseline hepatitis B virus DNA level was 89 IU/mL (range, <34-807 IU/mL). Virological rebound (as defined by a 10-fold increase in serum hepatitis B virus DNA level from pre-chemotherapy level persisted for 4 weeks) occurred in one of the 10 patients, followed by biochemical reactivation. Whereupon entecavir treatment was started and no liver failure ensued. Regarding the other seropositive occult patients, their serum hepatitis B virus DNA levels fluctuated, but there was no associated biochemical reactivation. CONCLUSION. Detectable baseline serum hepatitis B virus DNA is not uncommon in patients with occult hepatitis B who receive chemotherapy. Transient elevation in serum hepatitis B virus DNA levels does not predict biochemical reactivation, but antiviral treatment might be considered if virological rebound persists.