We evaluated the ability of intravenous supplementation therapy with α₁-antitrypsin (AAT) to reduce the rate of urinary excretion of desmosine (DES), a specific marker of elastin degradation, in eight men and four women with emphysema due to severe, congenital deficiency of AAT (range 17–69 mg/dl). Nine were former cigarette smokers, two were current smokers, and one reported never smoking; their mean age was 54 (SD 12) yr and their mean FEV₁ was 41 (18%) of predicted. Urinary DES was measured by isotope dilution and HPLC. Prior to the start of AAT supplementation, mean DES excretion was 13.0 (5.0) μ g/g creatinine, 73% higher than in healthy nonsmokers. During 8 wk of supplementation therapy, mean urinary DES excretion was 13.0 (5.9) μ g/g creatinine, unchanged from the baseline period (p = 0.85 by repeated measures ANOVA). We conclude that baseline levels of elastin degradation in emphysematous patients with severe AAT deficiency were abnormally high and that 8 wk of AAT supplementation therapy did not appreciably reduce the rate of elastin degradation. These findings raise the possibilities that protective levels of AAT in the lungs are insufficient or that elastin degradation in the lungs of these subjects is not dependent upon neutrophil elastase at this time.