In patients with chronic HBV infection, immunosuppressive therapy leads to the loss of immune control of replication and excessive increase of HBV viremia. Reactivation of HBV is usually characterized by ALT activity elevation but it may also cause acute liver failure. In patients with a past history of HBV infection (anti-HBc positive), the virus persists in liver cells and may lead to HBV recurrence with identical manifestation. Reactivation and recurrence of hepatitis B may be induced not only by immunosuppressive therapy, but also by anti-CD20 or anti-TNFa treatment. In inactive HBsAg carriers, HBV recurrence and reactivation may be prevented by prophylactic antiviral therapy with lamivudine or other synthetic antivirals (tenofovir, entecavir). Patients who underwent HBV infection in the past should be closely monitored and the treatment should be started when viremia increases. In chronic HCV infection, viremia also increases due to immunosuppression, progression of liver fibrosis is accelerated, but fulminant liver failure is rare. In HCV infected patients, preemptive antiviral treatment cannot be used. In immunocompromised patients, therapy with peginterferon alpha should be indicated individually. Kidney transplant candidates should undergo antiviral treatment before kidney transplantation, during the hemodialysis period.