There is a progressive decline in carbohydrate tolerance associated with CF genotypes whose major manifestations include pancreatic insufficiency. During the long silent period prior to the development of overt diabetes mellitus, these alterations may impair the patients' ability to maintain optimal nutrition, growth and pulmonary function. We will test the hypothesis that three years of sulfonylurea therapy will improve the nutritional status, promote growth, and slow the progressive decline in pulmonary function in patients with CF who are not yet insulin-requiring diabetics.