Parvovirus B19 infection is associated with a broad spectrum of clinical manifestations among which some are well known but others remain controversial. The role of this infection as a cause of acute hepatitis or exacerbation of chronic liver disease requires discussion regarding its significance in a strategy of prevention and treatment of patients with chronic hepatitis. Clinical importance of this infection in patients with chronic hepatitis B treated with pegylated interferon alpha 2a is still unclear but exactly in this population significant complications during treatment may arise. Parvovirus B19 infection is not rare among persons with chronic hepatitis B, therefore searching for co-infection should be placed in standard diagnostic procedures especially in case of exacerbation of chronic hepatitis, pancytopaenia or anaemia of unknown origin. Pegylated interferon alpha 2a still remains a gold standard of therapy of patients with chronic hepatitis B according to European (EASL) and Polish guidelines. We present a case of 35 years old woman treated with pegylated interferon alpha 2a who developed acute liver failure in 23rd week of chronic hepatitis B therapy. An exacerbation of hepatitis with encephalopathy and pancytopaenia have been observed. Parvovirus B19 and HBV co-infection does not increase the frequency of liver function abnormalities in patients with chronic hepatitis B. Further investigations should be done to describe the natural course of co-infection with parvovirus B19 and HBV and to establish possible association between parvovirus B19 infection and chronic hepatitis B and also the influence of interferon alpha 2a on the infections course.