Multiple breath washout (MBW) is a sensitive technique that detects early airways disease. However in very young children, large equipment and physiological dead space relative to lung volumes may result in a higher Lung Clearance Index (LCI). We investigated whether alveolar LCI (aLCI) is a more sensitive index than standard LCI in children. MBW data-sets from children aged 0.1-10.7 years [97 healthy controls and 93 with cystic fibrosis (CF)] were analysed. LCI is traditionally calculated by dividing the cumulative expired volume (CEV) by functional residual capacity (FRC) after correcting for equipment dead space. aLCI was calculated similarly, but after correcting the CEV and FRC for Langley's physiological dead space. There was a significant correlation between LCI and aLCI in health (r(2): 0.993; p<0.0001) and disease (r(2): 0.984; p<0.0001). Sensitivity of both LCI and aLCI in detecting abnormal lung function in CF was 39% during infancy, which increased to 77% and 83% respectively in older children. However, the difference in sensitivity (aLCI vs. LCI) was not significant (p=0.36). We conclude that LCI is minimally affected by airway deadspace, or relative equipment deadspace, and is an appropriate measure of lung function in infancy.