As a result of the improvement in life-expectancy in cystic fibrosis patients, simultaneous presence of cystic fibrosis and diabetes mellitus is no longer exceptional. In teenagers and young adults with cystic fibrosis, the prevalence of insulin-dependent diabetes mellitus (IDDM) is 7 to 10%. Fifty percent of cystic fibrosis patients have impaired glucose tolerance. These prevalences increase with advancing age. Insulin deficiency is a consistent feature. An endocrine pancreatic deficiency thus exists in addition to the exocrine pancreatic deficiency, as demonstrated by the fall in glucagon and pancreatic polypeptide productions. Development of insulin dependency is associated with deterioration in clinical status and indicates an adverse prognosis. Although in cystic fibrosis patients diabetes mellitus seems to occur as a result of different pathophysiologic mechanisms than those involved in autoimmune IDDM, the risk of degenerative complications is similar in both conditions. It follows that early detection of diabetes mellitus and appropriate insulin treatment are warranted in cystic fibrosis patients.