The evaluation of airway reactivity plays a central role in the diagnosis of bronchial hyperreactivity and asthma. The authors used high-resolution computed tomography (HRCT) to assess airway reactivity and compared the results with simultaneously performed measurements of airway pressure (Paw). Ten anesthetized and ventilated dogs were studied in a control state, after saline aerosol application, and after histamine aerosol challenge. In each condition, Paw was determined and HRCT was performed at functional residual capacity. On the HRCT scans, the cross-sectional areas of airway lumina were measured by using a computer edging process. After histamine challenge, HRCT demonstrated a decrease in airway areas of 43% +/- 2% (mean +/- standard error) from baseline (control) and Paw increased 99% +/- 18%. Surprisingly, saline aerosol challenge also resulted in a significant decrease in airway areas (26% +/- 3%) from control, while Paw measurements did not change significantly. Airway reactivity varied between dogs and within dogs. The authors conclude that HRCT can depict the site and degree of airway reactions and thus represents a new tool to assess airway reactivity in vivo.