The majority of cystic fibrosis (CF) patients now survive beyond childhood, and CF related diabetes (CFRD), due to insulin deficiency, is common. CFRD with fasting hyperglycemia occurs in about 15% of adult CF patients. Standard insulin therapy has relied primarily on meal coverage with rapid-acting insulin. Usually, basal insulin coverage is only provided overnight, with modest doses of NPH insulin. The practice of providing minimal basal insulin in CFRD is based on the fact that most of these patients, unless they are acutely ill, are able to maintain relatively normal blood glucose levels during the day without it. In addition, anecdotal experience has suggested that daytime NPH insulin or once to twice daily ultralente insulin frequently lead to hypoglycemia in the CFRD patient. This practice, which is based on practical clinical considerations, ignores the established relationship between insulin deficiency and clinical deterioration in CFRD. BMI and pulmonary function deteriorate much more rapidly in CF patients with diabetes than in CF patients with normal glucose tolerance. Insulin deficiency leads to increased protein catabolism and fatty acid turnover. The resulting loss of weight and lean body mass contributes to pulmonary disease and clinical decline.