Reactivation of hepatitis B virus (HBV) is a frequent complication of chemotherapy (CT) in patients with HBsAg carriers. In this prospective study, we documented CT induced HBV reactivation risk in patients with hematological malignancies. HBV reactivation risk is influenced by baseline viral load. Therefore, we divided our study population into two groups according to HBV-DNA status. HBV-DNA negative patients (n=18) were treated with nucleoside analogues once HBV reactivation was observed. HBV-DNA positive patients (n=12) commenced lamivudine before the initiation of the CT. In HBV-DNA negative patients HBV reactivation was found in 10 patients (55.5%). HBV reactivation was significantly more frequent in chronic lymphocytic leukemia (CLL) patients (P=0.008) and in patients receiving rituximab containing chemotherapy regimens (P=0.06). Eight patients (80.0%) responded to antiviral treatment after HBV reactivation. Two CLL patients experienced a flare-up after the withdrawal of antiviral therapy. In HBV-DNA positive patients, HBV reactivation was observed in four patients (33.3%) during lamivudine treatment and in two patients after lamivudine withdrawal. This study demonstrated the increased risk of CT-induced HBV reactivation in CLL patients, for the first time.