Ivacaftor is gene-specific oral therapy for patients with cystic fibrosis who have a cystic fibrosis transmembrane conductance regulator mutation, G551D. To date, limited information is available about the benefit in patients with severe CF related lung disease, as such patients were excluded from the phase III trials. We report the early results on clinical outcomes, sweat electrolytes and C-reactive protein in three adults with a G551D mutation and advanced lung disease. A mean increase of 6% in FEV1 was observed at 2 weeks and a mean reduction in sweat chloride of −48.9 mmol/L. While improvements in spirometry, weight gain and reduction in sweat electrolytes are similar with those reported in the phase III trials, a formal comparison was not performed.