BACKGROUND: Asthmatics have accelerated lung function decline over time compared with healthy individuals. OBJECTIVE: To evaluate risk factors for accelerated lung function decline. METHODS: In a longitudinal analysis on severe asthmatics enrolled in the Belgian Severe Asthma Registry with at least 2 visits a minimum of 12 months apart, we compared characteristics of patients with and without decline (loss of post-bronchodilation forced expiratory volume in 1 s [FEV1] (% predicted)/y greater than zero) over time. Multiple linear regression was applied to study the factors independently associated with FEV1 decline. RESULTS: In the overall population (n = 318), median annual FEV1 decline was 0.27 (-4.22 to 3.80) % predicted/y over a period of 23 months (12-41 months). Asthma was less controlled at baseline in nondecliners than in decliners (53%). Lung function and residual volume at baseline were higher in the declining group. Decliners presented with increased bronchial reactivity (ie, a lower provocative concentration of methacholine causing a 20% fall in FEV1) at baseline. Twenty-five percent of nondecliners were started on anti-interleukin-5 (anti-IL-5) for severe eosinophilic asthma during the study compared with 10% of decliners. The multivariable model suggested that Asthma Control Questionnaire score at baseline, late-onset asthma, and addition of anti-IL-5 during follow-up were associated with lower FEV1 decline, independently from other variables such as evolution in exacerbations, smoking status, inhaled corticosteroids or oral corticosteroids dose, or add-on anti-immunoglobulin E over time, whereas reversibility to salbutamol and higher FEV1 were associated with accelerated FEV1 decline. CONCLUSIONS: Add-on therapy with anti-IL-5 in severe eosinophilic asthma was associated with an attenuated FEV1 decline. The causality of this observation should, however, be confirmed in future prospective controlled studies.