Diabetes mellitus in cystic fibrosis (CF-DM) is increasingly prevalent with age, develops insidiously and is characterized by insulinopenia, normal insulin sensitivity, increased insulin clearance rate and concomitant exocrine pancreatic insufficiency. Since CF-DM impairs overall CF clinical status, including lung function, and may result in late diabetic complications, the condition should be screened for by annual oral glucose tolerance tests (OGTTs) from the age of 10 years, and be treated with insulin from the time of diagnosis of CF-DM.