Background: There is currently limited information regarding lung clearance index (LCI) and its response to treatment of pulmonary exacerbations in CF. We aimed to examine the utility of LCI for assessing short term clinical response to IV antibiotic therapy in school-age children with CF. Methods: Subjects experiencing exacerbations and hospitalised for IV antibiotics performed both multiple breath nitrogen washout (MBNW) and spirometry on admission to hospital and prior to discharge. Results: 27 patients (aged 6-20 years) had paired data for MBNW and spirometry. Mean LCI reduced from 12.18 to 11.65 (4.4%) by time of discharge and FEV1 z-score improved from -3.05 to -2.86 (6.2%). Overall, LCI improved in n = 15 (55%) patients compared with n = 18 (67%) where FEV1 improved. Conclusions: In summary, these findings do not support the use of LCI (or indeed, FEV1) to gauge the short term clinical response to IV antibiotic therapy in school-age children with cystic fibrosis. Crown Copyright (C) 2014 Published by Elsevier B.V. on behalf of European Cystic Fibrosis Society. All rights reserved.