α1-Antitrypsin deficiency, which can lead to both emphysema and liver disease, is a result of the accumulation of α1-antitrypsin polymers within the hepatocyte. A wealth of biochemical and biophysical data suggests that α1-antitrypsin polymers form via insertion of residues from the reactive center loop of one molecule into the β-sheet of another. However, this long-standing hypothesis has not been confirmed by direct structural evidence. Here, we describe the first crystallographic evidence of a β-strand linked polymer form of α1-antitrypsin: the crystal structure of a cleaved α1-antitrypsin polymer.