In late August, the Irish Blood Transfusion Service (IBTS) announced that a recipient of a blood transfusion had been infected with hepatitis B by that blood transfusion. Hepatitis B is a common form of infectious hepatitis, causing millions of infections worldwide every year. Transmission is usually through hepatitis B infected blood and body fluids. Transmission of hepatitis B via blood transfusion is now a very rare event. There are two separate tests carried out on each donor including a very sensitive nucleic acid test (NAT) for actual virus. Following infection with hepatitis B, an individual may not produce antibodies to the virus which can be detected for up to three months after infection. By using individual donor NAT testing, the presence of the virus can be detected in the infected individual about fifteen days after infection. However, for the first fifteen days after an individual is infected, their blood will not test positive for hepatitis B antibody or virus using NAT. This is the so called “window period” when transmission via blood can occur. It is an extremely rare event with the transmission of hepatitis B having an expected prevalence in Ireland of one in every two million donations. Since the introduction of NAT testing, the IBTS have tested more than 1.2 million donations. This is the first case of a confirmed window period donation known to result in a hepatitis B infection in a blood recipient in Ireland. This is a very unfortunate event for the individual involved. All the tests and procedures appear to have been properly carried out but this demonstrates that even with the most sophisticated and up to date testing regimes, there remains a very low risk of transmission of viruses when the donor is in the brief window period, between being infected and having any signs of infection in their blood. It confirms the continued importance of vigilance, of stringent donor deferral policies and of the development and future implementation of pathogen inactivation techniques for blood, platelets and other labile components. For those people who use plasma derived factor concentrates, hepatitis B is a lipid enveloped virus which is very effectively inactivated by the current viral inactivation methods used for factor concentrates. There has not been a reported infection with hepatitis B via clotting factor concentrates since the 1990’s.