We report the unique occurrence of an allogenic bone marrow transplantation performed as the donor was suffering from an acute hepatitis A. The bone marrow was contaminated at the time of collection, as demonstrated by hepatitis A virus (HAV) RNA detected by RT-PCR. Hepatitis A virus infection in such a situation could have resulted in a severe liver disease in the recipient. However, although we could demonstrate that the recipient had been infected, he did not develop a symptomatic hepatitis A but only minor disturbances of liver function tests between days 35 and 55. Both the postponement of the transplantation and the use of intravenous polyvalent immunoglobulins have probably played a key role in decreasing the viral load and allowing a rapid clearance of the virus. A possible role of the grafted immune system might also be envisaged, as suggested by the de novo synthesis of IgM in the recipient.