 Patients with severe asthma represent the greatest unmet medical need among the asthmatic population, in particular due to their high risk of severe exacerbations. A substantial proportion of these patients does not achieve asthma control despite even high-dose ICS and LABA treatment, necessitating add-on therapy. For these patients oral corticosteroids OCS are a preferred treatment modality, which will inevitably lead to severe side effects. A high Proportion of patients with severe Asthma are Vitamin D insufficient 30 ngml. It is therefore tempting to speculate that vitamin D substitution in vitamin D insufficient patients who continue to experience suboptimal asthma control on ICS and LABA or ICSLABA plus OCS treatment may boost corticosteroid responsiveness, thereby potentially reducing corticosteroid dose requirements and improving key parameters of asthma control. 