 The present study is a multicenter, randomized, double-blind, placebo-controlled study to evaluate the efficacy and safety of treatment with vitamin D at a dose of 4000 IUday administered daily following a single loading dose of 100 000 IU upon study entry over a period of 24 weeks in patients with severe asthma and vitamin D insufficiency. During the study, all patients will remain on their existing maintenance asthma therapy throughout the study, in addition to the study treatment, while reducing inhaled or oral steroids according to study protocol. The study consists of a 2-week run-in period, a 24 week double-blind treatment period including a 12 week steroid stable phase and two steroid reduction phases, followed by a follow-up visit 4 weeks after last study medication intake. The active treatment arm in this study will be standard of care plus vitamin D loading dose of 100 000 IU upon study entry, to be followed by 4000 IUday for the rest of the study period while the comparator arm will be standard of care plus placebo. 