Alpha 1-protease inhibitor can exist as over seventy different biochemical variants (the Pi system) which are inherited as autosomal codominant alleles. The majority of these variants are of no clinical significance. Epidemiologically, the most abundant are Pi types M, S and Z. Homozygotes of type Z have only 10 to 20% of the normal serum concentration of the inhibitor and have an increased risk of developing pulmonary emphysema. Cigarette smoking is the most important risk factor. A minority of Pi Z homozygotes (10 to 20%) develop a form of neonatal hepatitis and a proportion of these suffer from cirrhosis in adult life. Heterozygotes of Pi type SZ have about one third of the normal serum alpha 1-protease inhibitor concentration but this phenotype does not in itself appear to be a significant emphysema risk factor. Heterozygotes of Pi type MZ are thought to have a moderately increased risk of developing emphysema but only if they smoke; there is also evidence for an increased risk of cirrhosis among subjects of type MZ. No excessive risk appears to be attached to the MS phenotype. Cumulative survival curves have suggested that type Z homozygotes have a poor prognosis but such estimates are based on clinic or hospital patients who already have respiratory symptoms. Calculations based on population frequencies however, suggest that about 90% of the total number of type Z subjects are not accounted for in such surveys. Their whereabouts remains unclear at present; some will undoubtedly have died of liver or lung disease but it is possible that the majority escape and live undetected among the general population.