Since Wands et al. reported for the first time in 1975 the reactivation of the hepatitis B virus in hematologic disease patients who had been receiving chemotherapy, the efficacy of chemotherapy and immunosuppressants has improved. As a result, the frequency of the reactivation of hepatitis B is increasing. Reported herein is a case of a non-Hodgkin lymphoma patient in her 70s who was suspected to have had HBsAg negative/anti-HBs negative occult HBV infection. The patient experienced fulminant hepatitis caused by the reactivation of hepatitis B, and died three months after the R-CHOP regimen was completed. In the HBsAg negative plus HBV DNA-negative case, there were few instances of viral activation of HBV. In this case, antiviral therapy was needed when the patient was confirmed to have become HBV DNA positive through regular monitoring, but its necessity is often overlooked, unlike the preemptive antiviral treatment in the HBsAg positive cases.