We examined the role of intermediate alpha 1-antitrypsin deficiency in predisposing to abnormalities of lung distensibility and airway function in 20 heterozygotes (Pi MZ) who were individually matched with a control Pi M subject of similar age, height, and smoking habits drawn from the same male, working population. There were no significant differences between the heterozygotes and their controls in the results of spirometry, maximum expiratory flow-volume curves (breathing air), single breath nitrogen test, arterialised capillary blood oxygen pressure, or single breath carbon monoxide transfer. Additional studies were made in 12 of the pairs of Pi MZ and Pi M subjects. Comparison of maximum expiratory flow-volume curves breathing air and 80% helium-20% oxygen showed no differences between the Pi MZ and Pi M subjects. Although airway function was similar in the two groups, four of 12 Pi MZ subjects showed abnormalities of the pressure-volume curve of the lung (reduction in lung recoil pressure, abnormal shape factor, increase in functional residual capacity). Abnormalities of washout of a helium-sulphur hexafluoride gas mixture, of a type previously described as characteristic of emphysema, were found in two of the men with abnormal pressure-volume curves. The results suggest that Pi MZ subjects have an increased susceptibility to alveolar abnormalities without increased abnormalities of airway function; this may explain the increased frequency of emphysema at necropsy despite many studies showing no predisposition to abnormal airway function in life. The functional changes we observed would be unlikely to cause symptoms. The risk of disablement from chronic lung disease appears to be only slightly enhanced by intermediate alpha 1-antitrypsin deficiency.