Patients with severe hereditary alpha1-antitrypsin deficiency (alpha1-ATD) face a high risk of developing emphysema at a young age. Intravenous augmentation therapy with purified human alpha1-antitrypsin (alpha1-AT) is now available. However, a controlled trial to show its efficacy has never been carried out. The aim of this study was to compare the decline in forced expiratory volume in one second (deltaFEV1) between Danish patients who had never received augmentation therapy and German patients treated with weekly infusion of alpha1-AT. From the files of the Danish alpha1-ATD register, 97 exsmokers, with a PiZ phenotype and for whom results of at least two lung function measurements with an interval of at least 1 yr were available, were identified. From a German group of patients treated with weekly infusions of alpha1-AT, 60 mg x kg(-1) body weight, 198 exsmokers, with biannual lung function measurements were identified. The deltaFEV1 was compared between the two treatment groups by random effects modelling. The deltaFEV1 in the treated group was significantly lower than in the untreated group, with annual declines of 53 mL x yr(-1) (95% confidence interval (95% CI) 48-58 mL x yr(-1)) and 75 mL x yr(-1) (95% CI 63-87 mL x yr(-1)), respectively (p=0.02). The two groups differed with respect to gender and initial FEV1% predicted. Gender did not have any influence on the deltaFEV1. Stratification by initial FEV1% pred showed a significant effect of the treatment only in the group of patients with an initial FEV1% pred of 31-65%, and deltaFEV1 was reduced by 21 mL x yr(-1). This nonrandomized study suggests that weekly infusion of human alpha1-antitrypsin in patients with moderately reduced lung function may slow the annual decline in forced expiratory volume in one second.