A case of de novo acute hepatitis B that showed symptoms of general malaise and anorexia during rituximab therapy with the CHOP regimen for diffuse large B cell lymphoma is reported. Lamivudine was strikingly effective, showing a rapid recovery from liver damage with jaundice. Hepatitis B virus (HBV) DNA in serum became and stayed undetectable even after the withdrawal of lamivudine, although HBsAg remained positive over 42 months from the onset. Liver biopsy showed a picture suggestive of acute viral hepatitis with multinucleated giant hepatocytes and CD38-positive plasma cell infiltration into liver parenchyma. Immunohistochemically, CD3-positive T-cells were predominant cells that infiltrated in liver parenchyma, whereas CD20-positive B cells were essentially null. Hence, it is suggested from these findings that B lymphocytes might be crucial for the continuous latency in HBV infection and may give rise to de novo acute hepatitis B if totally deleted. Moreover, the CHOP regimen might have some additive effects with the repeated on-off use of corticosteroids to the onset of the disease. In addition, significance of plasma cell infiltration in this setting is discussed.