A morphometric pilot study was performed on airway smooth muscle in 20 autopsy lungs with no history of chronic obstructive pulmonary disease (COPD) to establish a basis for the systematic analysis of the behavior of airway muscles in bronchial asthma or COPD. A total of 514 samples of cross-sectioned airways were taken from these lungs, covering as uniformly as possible the whole range from segmental bronchi to terminal bronchioles. In each airway, the perimeter length L of the epithelial basement membrane and the area S of mural smooth muscles were measured using a digital image analyzer, and the thickness D of smooth muscles and the radius R were determined in a standardized state in which the airway was stretched into a circle without changing L or S. On bilogarithmic coordinates, D was found to correlate significantly with R not only in individual cases but also in the total pooled data. D/R, the relative thickness of muscles, increased toward the periphery of the airways. In a theoretical application of Laplace's law to the physical conditions dominating the airway wall, this tendency in D/R suggests that the activity of airflow regulation is highest in terminal airways.