The lungs of 6 elderly nonsmoking persons with severe long-standing "allergic" asthma (mean duration, 45 yr), not dying in status asthmaticus, were examined quantitatively to characterize the structural alterations that would produce clinical chronic air-flow obstruction. The percent of bronchial smooth muscle was not significantly elevated in these asthmatics, compared with that in 7 control subjects. Two asthmatics had reduced small airway diameters, with histologic evidence of inflammation or fibrosis. Mean linear intercept (interalveolar distance) in asthmatics was not significantly elevated over that in control subjects, but one asthmatic had a clearly increased mean linear intercept and histologic evidence of very mild emphysema. Bronchial basement membrane thickness was 8.3 +/- 2.0 mu in asthmatics and 5.1 +/- 0.9 mu in control subjects (p less than 0.01), a measurement that discriminated best between asthmatics and nonasthmatics. Small airway narrowing may develop in long-standing asthma to explain the component of nonreversible air-flow obstruction that characterizes such patients.