Immunosuppression because of local or systemic chemotherapy or immunosuppressive therapy of autoimmune diseases is an increasing risk for reactivation of hepatitis B and may lead to acute hepatitis and rarely to fulminant hepatitis. Intensive immunosuppression, e. g. in advance of bone marrow transplantation, may lead to re-seroconversion with loss of anti-HBs and detection of HBsAg and HBV-DNA and a risk of (fulminant) hepatitis. Distinct chemotherapeutics and immunosuppressive medication promote reactivation or re-seroconversion of HBV infection. Several studies have shown a significant benefit by antiviral therapy with nucleosidanalogue lamivudine to avoid reactivation. For other nucleoside analogues (telbivudine, entecavir) or nucleotide analogues (adefovir dipivoxil, tenofovir) larger experiences are yet missing in this situation. Prophylaxis with antiviral agents may be superior to therapy of reactivation. Prior to onset of immunosuppressive treatment patients must be tested for HBV. Patients without detection of HBsAg and without sufficient anti-HBs-titer should receive active immunisation. In case of chronic HBV infection antiviral therapy should be initiated before intense immunosuppression. In case of HBV infection in medical history (anti-HBc positive, HBsAg negative) HBV serostatus should be tested frequently. In case of reactivation, therapy with nucleos(t)ide analogue should immediately be initiated. In case of bone marrow transplantation prophylaxis with nucleos(t)idanalogue has to be initiated before onset of treatment if anti-HBc is detectable, independent from HBsAg result.