 At baseline, all patients will undergo a mannitol provocation test and the fraction of exhaled nitric oxide FeNO will be measured. If positive, and at least two days later, a methacholine provocation test and repeated measurement of FeNO will be performed. For standardization patients will be prescribed to standardized ICS LABA therapy in a fixed combination with budesonide formoterol 200 6 μg bid. After a two-week run-in period on this therapy, 10mg montelukast will be added for a 4 weeks treatment duration interim visit. All patients included will attend the final visit in which provocation tests were repeated. 