Airway inflammation is a common feature of asthma, and one of the cardinal features of inflammation is increased microvascular permeability. We investigated the characteristics of inhaled ovalbumin challenge-induced airflow obstruction and airway microvascular leakage in vivo in mechanically ventilated guinea pigs actively sensitized to ovalbumin. A method was used to quantify both airflow obstruction and airway microvascular leakage in order to investigate the relationship between these 2 pathophysiological features in the same animal. Airway microvascular leakage was assessed by Evans blue dye extravasation into airway tissues. Actively sensitized guinea pigs developed both acute airflow obstruction (increased lung resistance and reduced dynamic lung compliance) and Evans blue dye extravasation in response to exposure to aerosolised ovalbumin. Evans blue dye extravasation was preferentially distributed in the distal airways and correlated with airflow obstruction. The results show that inhaled allergen induced both acute airflow obstruction and airway microvascular leakage.