Posttransfusion purpura is an increasingly rare complication of blood transfusion. It is characterized by an acute episode of severe thrombocytopenia occurring about a week after a transfusion in HPA-1a negative women who have been alloimmunized by pregnancy. Although the thrombocytopenia resolves spontaneously within a few weeks, haemorrhage is common and sometimes severe. High-dose intravenous immunoglobulin (IVIgG) (2 g/kg given over 2 or 5 days) is the current treatment of choice to shorten the duration of thrombocytopenia, with responses in about 85% of cases. The pathophysiology remains uncertain. Universal leucocyte reduction of blood components in the UK has resulted in a marked reduction in the number of reported cases. This edition first published 2013 © 2001, 2005, 2009, 2013 John Wiley & Sons Ltd.