Adult respiratory distress syndrome (ARDS) is a rare but important complication of blood transfusion because it has a mortality rate of 50-60%. ARDS is characterised by noncardiogenic pulmonary oedema and is often associated with major trauma and/or sepsis. Clinical features include dyspnoea, tachypnoea, chills and extensive crepitations. The pathogenesis has not been elucidated completely and a number of hypotheses have been proposed. Factors which have been implicated include neutrophil sequestration and complement activation, macrophages, metabolites of the arachidonic acid cascade and cytokines, all of which contribute to the amplification of the inflammatory process. In particular, leucoagglutinins have been implicated with blood transfusions. Treatment is generally supportive as specific therapeutic strategies remain largely unproven.