Reactivation of hepatitis B virus (HBV) during chemotherapy is well documented. However, there are limited data on this complication in patients with hepatocellular carcinoma (HCC) undergoing transarterial chemotherapy. The aim of this study was to evaluate the efficacy of preemptive lamivudine therapy in reducing hepatitis due to HBV reactivation in patients with HCC undergoing transarterial chemo-lipiodolization (TACL) and to seek predictors of this event. A total of 73 consecutive HCC patients undergoing TACL using epirubicin 50 mg/m2 and cisplatin 60 mg/m2 at monthly intervals were prospectively and randomly assigned to receive lamivudine 100 mg daily from the start of TACL (preemptive group) or not (control group). During the study, 11 (29.7%) of 37 patients in the control group and 1 (2.8%) of 36 patients in the preemptive group developed hepatitis due to HBV reactivation (P = .002). In addition, there were significantly more incidences of overall hepatitis (P = .021) and severe grade of hepatitis (P = .035) in the control group. With multivariate Cox regression model, a baseline HBV DNA level of more than 10(4) copies/mL was the only independent predictor of hepatitis due to HBV reactivation during chemo-lipiodolization (P = .046). In conclusion, preemptive lamivudine therapy demonstrated excellent efficacy in reducing hepatitis due to HBV reactivation and hepatic morbidity during TACL. Preemptive therapy should be considered in HCC patients with an HBV DNA level of more than 10(4) copies/mL. Further studies are needed to confirm the value of this approach in patients with low-level viremia.