In order to study whether hyperplasia or hypertrophy of cells is responsible for the thickening of airway muscles, 3-D morphometry of airway muscle cells was performed on resin-embedded semithin serial sections of autopsied lungs from 10 asthmatics and five control subjects. There were five Type I and five Type II asthmatic lungs, as defined in an earlier study, thickened muscles being found only in the central bronchi in Type I and distributed over the whole airway tree in Type II. The analysis was based on "unbiased" 3-D morphometry to obtain the numerical density NV of muscle cells using a "disector," a spatial probe introduced by Sterio in 1984, which we modified into a stack of serial sections. The mean number NL of cells per unit airway length and the mean volume Vc of a single muscle cell were also determined. In Type I asthmatics, the number of cells increased in the larger bronchi unaccompanied by cellular hypertrophy at any level of the airway tree. In contrast, in Type II asthmatics, hypertrophy was shown to prevail over the whole airway, but it was most remarkable in the bronchioles, whereas hyperplasia was mild and localized only in the bronchi. The two types of asthmatic lungs may therefore result from different pathogeneses.