Mutations in the alpha 1-antichymotrypsin gene have been described which result in reduced levels of alpha 1-antichymotrypsin in the serum. Previous studies have suggested that two of these mutations (Pro227-->Ala and Leu55-->Pro) predispose to chronic obstructive pulmonary disease (COPD). We have investigated the prevalence of these mutations in 168 COPD patients and 61 controls without airflow obstruction. The prevalence of the Pro227-->Ala mutation was 0.9% and it was not associated with impaired lung function. None of the subjects had the Leu55-->Pro mutation.