Alpha 1 antitrypsin (AAT) deficiency (AATD) is a rare genetic risk factor that predisposes an individual to develop early-onset emphysema. While accurate diagnosis of severe AATD is an important goal of clinical care, a minority of individuals with AATD are diagnosed and lack of awareness about the disease is the major suspected cause for this. Since the 1980s, therapy with human plasma-derived AAT has been the only specific treatment for AATD aiming to slow emphysema progression. The first randomised controlled trial to demonstrate this slowing of disease progression with AAT was the Randomized, placebo-controlled trial of augmentation therapy in Alpha1 Proteinase Inhibitor Deficiency (RAPID) study. The RAPID programme, consisting of the initial trial plus its open-label extension (OLE), is the largest completed clinical study of AAT therapy in AATD and the only trial designed specifically to explore the disease-modifying effect of AAT treatment. The RAPID-OLE data substantiate those of the RAPID trial, establishing the sustained efficacy and good tolerability for AAT treatment, providing evidence that AAT treatment modifies the disease course, and supporting the importance of early intervention.