BACKGROUND The development of ivacaftor represents a significant advance in therapeutics for patients with cystic fibrosis (CF) who carry the G551D mutation. Patients with an FEV1 < 40% predicted represent a considerable proportion of eligible patients but were excluded from phase 3 clinical trials, and the effectiveness of the drug in this population is, therefore, unknown. METHODS Data were collected from adult CF centers in the United Kingdom and Ireland with patients enrolled in an ivacaftor compassionate use program (FEV1 < 40% or on lung transplant waiting list). Clinically recorded data were collated from patient records for 1 year prior and for a period of 90 to 270 days following ivacaftor commencement. Each patient was matched to two control subjects who would have met the requirements for the compassionate use program with the exception of genotype. RESULTS Twenty-one patients received ivacaftor for a median of 237 days. Mean FEV1 improved from 26.5% to 30.7% predicted ( P = .01), representing a 16.7% relative improvement. Median weight improved from 49.8 to 51.6 kg ( P = .006). Median inpatient IV antibiotic days declined from 23 to 0 d/y ( P = .001) and median total IV treatment days decreased from 74 to 38 d/y ( P = .002) following ivacaftor. Changes in pulmonary function and IV antibiotic requirements were significant compared with control subjects. CONCLUSIONS Ivacaftor was clinically effective in patients with CF who carry the G551D mutation and have severe pulmonary disease. The reductions in treatment requirements were clinically and statistically significant and have not been described in less severe populations.