A blinded, placebo-controlled, clinical trial to examine the effect of and oral glucose medication on growth, metabolism, and quality of life in cystic fibrosis. Twenty-four children will be recruited and subsequently randomly assigned to receive either Tolbutamide or placebo for a period of three months. All subjects are then crossed-over to the other agent for the next three months. Testing involves meal stimulation tests, isotope studies, quality of life assessments, and urine/blood specimen collection.