A patient with stage IV non-Hodgkin's lymphoma and chronic hepatitis who was treated with chemotherapy and rituximab developed fatal reactivation of hepatitis B. High viral load titers of hepatitis B DNA were demonstrated. DNA sequencing analysis revealed the presence of the precore mutant strain. Hepatitis B reactivation in patients receiving rituxmab-based chemotherapy is reviewed and the role of precore mutant virus strains resulting in fatal complications is discussed.