It has long been assumed that the ventilation heterogeneity associated with lung disease could in itself affect the measurement of carbon monoxide transfer factor. The aim of this study was to investigate the potential estimation errors of carbon monoxide diffusing capacity (DLco) measurement that are specifically due to conductive ventilation heterogeneity,i.e.,due to a combination of ventilation heterogeneity and flow asynchrony between lung units larger than acini. We induced conductive airway ventilation heterogeneity in 35 never-smoker normal subjects by histamine provocation,and related the resulting changes in conductive ventilation heterogeneity (derived from the multiple breath washout test) to corresponding changes in diffusing capacity,alveolar volume and inspired vital capacity (derived from the single breath DLco method). Average conductive ventilation heterogeneity doubled (p<0.001),while DLco decreased by 6% (p<0.001),with no correlation between individual data (p>0.1). Average inspired vital capacity and alveolar volume both decreased significantly,by respectively 6% and 3%, and the individual changes in alveolar volume and in conductive ventilation heterogeneity were correlated (r=-0.46;p=0.006). These findings can be brought in agreement with recent modelling work, where specific ventilation heterogeneity resulting from different distributions of either inspired volume or end-expiratory lung volume have been shown to affect DLco estimation errors in opposite ways. Even in the presence of flow asynchrony,these errors appear to largely cancel out in our experimental situation of histamine-induced conductive ventilation heterogeneity. Finally,we also predicted which alternative combination of specific ventilation heterogeneity and flow asynchrony could affect DLco estimate in a more substantial fashion in diseased lungs,irrespective of any diffusion-dependent effects.