Computed tomography (CT) of the lungs makes it possible to study individual airways. A technique using high-resolution-CT (HRCT) has been developed to measure the internal size of the bronchi. Five normal subjects had a conventional HRCT scan at full, deep inspiration. The internal size of the airways from segmental to the sixth generation bronchi was measured using an original method of data analysis. The size of the airways ranged 0.5-50 mm2 (approximately 0.8-8 mm in diameter. The technique was found to be highly reproducible, making it possible to study the variation in airways calibre after provocative challenge or administration of bronchodilator in asthma.