Cystic Fibrosis (CF) is the most common life-threatening genetic condition affecting Australian children. As well as repeated lung infections, children with CF develop insulin deficiency and eventually diabetes. The CF-IDEA trial (Cystic Fibrosis - Insulin Deficiency, Early Action) will determine whether starting insulin treatment before the onset of diabetes (earlier than current practice) will improve the health of children with CF by improving body weight and lung function.