We investigated the axial dispersive effect of the upper airway structure (comprising mouth cavity, oropharynx and trachea) on a traversing aerosol bolus. This was done by means of aerosol bolus experiments on a hollow cast of a realistic upper airway model (UAM) and 3D computational fluid dynamics (CFD) simulations in the same UAM geometry. The experiments showed that 50ml boluses injected into the UAM, dispersed to boluses with a halfwidth ranging 80-90ml at the UAM exit, across both flow rates (250,500ml/s) and both flow directions (inspiration, expiration). These experimental results imply that the net halfwidth induced by the UAM typically was 69ml. Comparison of experimental bolus traces with a 1D Gaussian derived analytical solution resulted in an axial dispersion coefficient of 200-250cm²/s, depending on whether the bolus peak and its halfwidth, or the bolus tail needed to be fully accounted for. CFD simulations agreed well with experimental results for inspiratory boluses, and were compatible with an axial dispersion of 200cm²/s. However, for expiratory boluses, the CFD simulations showed a very tight bolus peak followed by an elongated tail, in sharp contrast to the expiratory bolus experiments. This indicates that CFD methods which are widely used to predict the fate of aerosols in the human upper airway, where flow is transitional, need to be critically assessed, possibly via aerosol bolus simulations. We conclude that, with all its geometric complexity, the upper airway introduces a relatively mild dispersion on a traversing aerosol bolus for normal breathing flow rates in inspiratory and expiratory flow directions.