Under the assumption that the more hyperreactive the bronchial muscles are, the greater their work hypertrophy, we analyzed the distribution of hypertrophic smooth muscles along airways to see where in the bronchial tree asthmatic constrictions mainly occur. Autopsy lungs from 16 patients with bronchial asthma, 13 with other COPDs, and 20 controls were submitted to morphometry of the bronchial muscles. In microscopic slides, cross sections of airways were taken from the segmental bronchi to the terminal bronchioles. The perimeter length L of the basement membrane and the area S of muscles were measured, and the anatomic radius R and the muscular thickness D were calculated in a standardized circular state, in which the basement membrane was stretched into a circle without changing L or S. On bilogarithmic coordinates of D and R on which data from the asthmatics were pooled, it was shown that hypertrophy of muscles was the most pronounced in larger bronchi where constriction was most likely to occur. Closer analysis of patients, however, revealed that besides this typical pattern, which we designated Type I asthma, there was a group of patients (Type II) in whom hypertrophy involved the entire range of airways, including the bronchioles, suggesting that the site of asthmatic response varies among patients. In nonasthmatic patients with COPD, only mild hypertrophy of muscles was found in the large airways, despite the presence of obstructive lesions mainly in the small airways.