The effects of hyperpnea on parenchymal lung mechanics are unknown, but they may contribute to the resultant airflow limitation commonly seen in asthma. To investigate these effects, we measured the following parameters in seven asthmatic and six normal subjects before and after 5 min of hyperpnea: specific conductance, upstream resistance, static compliance, the coefficient of retraction, lung volumes, lung hysteresis, and the ratio of maximal to partial flow rates (the M:P ratio, an indicator of the effect of deep inhalation on airflow, and a measure of relative airway and parenchymal hysteresis). In addition to a central effect on the airways, as shown by significant falls in specific conductance, hyperpnea in asthmatics, but not in normal subjects, resulted in significant increases in residual volume and pressure-volume hysteresis, suggestive of changes in parenchymal lung mechanics. The M:P ratio also increased in the asthmatics, consistent with greater increases in airway than in parenchymal hysteresis after hyperpnea. We conclude that hyperpnea has significant effects on the lung parenchyma that contribute to airflow limitation in asthmatics, and we hypothesize that these effects may be due to alterations in peripheral airway smooth muscle tone and surfactant function.