Background: Severe asthma is associated with an accelerated lung function decline which is likely attenuated by the addition of anti-nnterleukin-5 (IL-5) therapy in patients with severe eosinophilic asthma. Objective: To study the long-term impact of add-on therapy with anti-IL-5 on FEV ₁ in severe eosinophilic asthma patients. Methods: In this post-hoc analysis, we compared, through a linear mixed model, the evolution of pre-bronchodilation FEV ₁ expressed in %pred in a cohort of 50 patients with severe eosinophilic asthma treated with anti-IL-5 (1576 visits) before (median follow-up: 9.2 years) and after (median follow-up: 2.1 years (up to 6.8 years) anti-IL-5 therapy start. Results: FEV ₁ decline was observed before anti-IL5 start (-0.6 %pred.year ^-1, p<0.001). FEV ₁ improved significantly after anti-IL-5 start (+1.3 %pred.year ^-1 p<0.001; difference pre-post: p<0.001). A sustained improvement was observed in 31 patients deemed responders (+3.1 %pred.year ^-1, p<0.001; difference pre-post: p<0.001) vs a continuous decline in 19 patients considered as non-responders (-0.40 %pred.year ^-1, p=0.087; difference pre-post: p=0.097). Non-responders exhibited a higher prevalence of nasal polyposis, better asthma control and a trend towards higher exhaled nitric oxide values. Conclusion: This post-hoc analysis shows that add-on therapy with anti-IL-5 not only stems the accelerated decline in lung function but also makes it reversible in many severe eosinophilic asthma patients, leading to an estimated improvement of 11% FEV ₁ %pred after 3 years of treatment. Persistent improvement in lung function is therefore feasible in severe asthmatics and could be chosen as a lung function criterion to define remission of asthma under treatment.