The molecular structure and the serum levels of alpha 1-antitrypsin, the major antiprotease of human serum, are controlled by a series of codominant alleles at a single chromosomal locus, known as the Pi(protease-inhibitor) locus. The congenital deficiency of this inhibitor is known to be associated with the development of lung emphysema in early adulthood and chronic liver disease in childhood. Less frequent associations have been reported, such as rheumatoid arthritis, membranoproliferative glomerulonephritis and mosaicism for sex chromosomes. The identification of several suballeles of the Pi system, which was accomplished by means of a refinement of the isoelectric focusing technique, has promoted research concerning their possible pathogenic implications. The studies so far performed have often led to contradictory results, but nevertheless they strongly ascribe the property of controlling the quantitative levels of alpha 1-antitrypsin to certain M subtypes. Intermediate M3 subtype has recently been associated with the development of chronic obstructive lung disease in adulthood. Should this finding be confirmed by further evidence, a new approach to the prevention of lung disease could be considered, given that 30% of the individuals are carriers of the M3 suballele. In Italy, the incidence of congenital deficiency of alpha 1-trypsin appears to be greater in the northern regions, where 15-20 out of every 100,000 individuals are affected by the severe (ZZ) form of the deficiency.