Objective: To analyse the incidence of hepatitis B reactivation following treatment with irinotecan-based combined chemotherapy and the outcomes of patients with hepatitis B reactivation. Patients and Methods: A prospective phase II study investigating the response to irinotecan, leucovorin, and 5-fluorouracil in 34 patients with stage IV colorectal cancers. All patients had hepatitis B virus status checked at baseline. Results: Three patients were identified to be hepatitis B virus carriers. Two of the patients had hepatitis B reactivation after chemotherapy commenced, while the third patient, who was given lamivudine before commencement of chemotherapy, did not have any adverse hepatic event. Conclusions: In view of the increasing use of irinotecan as palliative treatment for advanced colorectal cancer, further studies on the probable causal relationship between irinotecan and reactivation of hepatitis B infection are recommended. The prophylactic use of lamivudine in patients at risk might be beneficial.link_to_subscribed_fulltex