Cystic fibrosis (CF) is a genetic disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR), a chloride channel. Most CF mutations either reduce the number of CFTR channels at the cell surface (synthesis or processing mutations) or impair channel function (gating or conductance mutations). Ivacaftor (Kalydeco, VX-770) is a novel, FDA approved new therapy that addresses CFTR dysfunctions in subjects with CF and "gating mutations", specifically; it potentiates CFTR channel function. For mutations like G551D that permit CFTR expression at the cell membrane but compromise its activity, Ivacaftor increases the probability that the channel is open and active. In previous randomized, double-blind, placebo controlled trials, Ivacaftor treatment resulted in clinically significantly improvements in pulmonary function, weight and body mass index (BMI), and significant decreases in sweat chloride reflective of increased CFTR activity. The improvements in lung function and weight occurred over the first 8 weeks of treatment, plateaued and were sustained over the 48 weeks of the trial. The mechanism for the rapid and sustained weight gain is not known. Several mechanisms are considered in this proposal which may result in improved energy balance and energy utilization, and weight gain. These include decreased resting energy expenditure, increased energy and fat absorption from the gut, improved pancreatic enzyme and pH secretion, and increased energy intake. Improvements in weight and BMI status are expected to result from this improvement in energy balance and utilization, with potential beneficial effects on muscle mass and function and quality of life.