Introduction: Multiple breath washout (MBW) is used for early detection of cystic fibrosis (CF) lung disease, with SF 6 MBW commonly viewed as the reference method.The use of N 2 MBW in infants and toddlers has been questioned for technical and physiological reasons, but a new correction of the N 2 signal has minimized the technical part. The present study aimed to assess the remaining differences and the contributing mechanisms for the differences between SF 6 and N 2 MBW,correctedsuch as tidal volume reduction during N 2 washout with pure O 2 .Method: This was a longitudinal multicenter cohort study. SF 6 MBW and N 2 MBW were performed prospectively at three CF centers in the same visits on 154 test occasions across 62 children with CF (mean age: 22.7 months). Offline analysis using identical algorithms to the commercially available program provided outcomes of N 2,original and N 2,corrected for comparison with SF 6 MBW.Results: Mean functional residual capacity, FRC N2,corrected was 14.3% lower than FRC N2, original , and 1.0% different from FRC SF6 . Lung clearance index, LCI N2,corrected was 25.2% lower than LCI N2,original , and 7.3% higher than LCI SF6 . Mean (SD) tidal volume decreased significantly during N 2 MBW corrected , compared to SF 6 MBW (−13.1 ml [−30.7; 4.6], p < 0.0001, equal to −12.0% [−25.7; 1.73]), but this tidal