(1) Deficiency of alpha AT is one of the most common hereditary diseases affecting Caucasians in Europe. The alpha 1AT protein is extremely pleomorphic, and around 90 variants due to mutations have been recognized. The prime functions of alpha 1AT is to inhibit neutrophil elastase, and a proportion of individuals who are deficient in alpha 1AT develop emphysema. The most common deficiency variant (Z) is also associated with liver disease. The main site of alpha 1AT synthesis is in the liver. Not all deficient individuals are affected by lung or liver disease, however, so that other factors (genetic and environmental) are clearly important. (2) Investigation of alpha 1AT status is essential in any child or adult presenting with chronic liver disease. The genetic cause cannot be identified clinically or by any other laboratory investigation. The diagnosis carries important prognostic consequences and is important for other family members. Patients with emphysema should have their Pi type determined, especially if they are under the age of 50, have never smoked or there is a suggestive family history. Asymptomatic individuals who are homozygous type Z should be referred to a chest physician for a clinical and radiological assessment together with lung function tests. (3) Several laboratory tests are available to detect alpha 1AT deficiency, and the choice of test(s) will depend on circumstances. Quantitation of the serum protein is simple and cheap. Because alpha 1AT is an acute phase protein, however, quantitation used in isolation may give false negative results which are clearly unacceptable, particularly in association with paediatric liver disease. Phenotyping by isoelectric focusing requires some experience in distinguishing SZ and ZZ phenotypes, and phenotyping should ideally be used in conjunction with quantitation because heterozygous null phenotypes may appear identical to homozygous normal phenotypes. (4) Prenatal diagnosis is usually performed by DNA analysis of CVS samples obtained at 11-13 weeks. Because of the risk that CVS samples might be contaminated by maternal tissue, assays which are less likely to detect minor contaminants are preferable. At present, use of DNA tests is confined to prenatal diagnosis, but the availability of simple tests and the possibility of unequivocal identification of S and Z alleles means that these tests are likely to find greater use in the near future.