alpha 1-antitrypsin phenotypes were determined in 906 adults from an entire community; 4.9 percent were heterozygotes (Pi MZ) for severe deficiency and 8 percent were heterozygotes (Pi MS) for mild deficiency. Lung function both in cross-section and over a period of three years was examined in those subjects with the two heterozygote deficiency phenotypes and compared with the lung function of those with the normal Pi M phenotype. FVC and FEV1 were standardised for age, sex, height, race, smoking habit and quantum, and respiratory symptoms. Using techniques of multivariate analysis and rigorous matched pairs there was no evidence of a deterioration in lung function in Pi MS or Pi MZ subjects. The tests used did not exclude the possibility of minor alveolar abnormalities but the results do allow the confident counselling of heterozygotes that they can look forward to full respiratory health.