There is a high prevalence of diabetes mellitus in patients with CF and this is likely to increase in the future as more patients are surviving into adult life. In view of this all CF clinics should routinely screen for diabetes mellitus. The prevalence of diabetes mellitus in adult CF patients is higher than in children, and the onset is commonly insidious. The diabetes seen in CF is not classical Type 1 diabetes or Type 2 diabetes and could more helpfully be called cystic fibrosis-related diabetes (CFRD). Treatment is by oral hypoglycaemic agents or insulin. It is not appropriate to control patients by diet alone. Dietary advice to CF diabetic patients is not the same as that given to non-CF diabetic patients. Microvascular complications have now been reported and careful monitoring of all CF patients with diabetes should be undertaken.