alpha(1)-Antitrypsin (AAT), a 52 kDa plasma protein, is produced mainly in the liver. It is the most abundant circulating serine proteinase inhibitor (serpin). It has also previously been called protease inhibitor to reflect its function as a general inhibitor of serine proteases. Its main physiological role is to inhibit neutrophil elastase and it contributes to the innate immune system as an anti-inflammatory protein. Severe AAT deficiency is most prevalent in northern Europeans affecting about 1 in 3000 of the population. AAT deficiency predisposes individuals who smoke to developing pulmonary emphysema in the fourth-fifth decade of adult life and to childhood cirrhosis in about 10% of cases, with the initial presentation being prolonged neonatal jaundice. The mean interval from presentation with symptoms to diagnosis in adults is about 8 years. The condition is under-recognised and under-diagnosed. The only effective current treatment for the severe liver disease that occurs in childhood currently is liver transplantation. Replacement therapy with purified AAT from human plasma is being used in clinical practice for the lung disease though it is not known whether this influences the outcome of this chronic condition. The liver pathology arises from intracellular polymerisation of mutant protein, and attenuation of polymerisation is a potential target for therapy.