Aims: to test the feasibility and safety of withdrawal of ICS in CF by performing a study of randomised placebo-controlled withdrawal of ICS in children and adults already taking them. The results will be used to determine the feasibility of a future prospective study to prove whether starting ICS in CF patients not already on them, and those successfully withdrawn, leads to benefit (in lung function and chest exacerbations) and/or harm, compared to placebo.