Bronchodilatation follows a deep inspiration in normal subjects with pharmacologically induced bronchoconstriction. To determine to what extent this occurs in asthmatic patients with spontaneous bronchospasm, we obtained partial (PEFV) and complete (MEFV) maximum expiratory flow volume curves in 20 adults (helium-oxygen responders and nonresponders) and 13 children with asthma. We measured maximum expiratory flow at 25 percent of forced vital capacity from the partial (Vmax - p) and complete (Vmax - c) flow-volume curves and expressed this relationship as the Vmax - p/Vmax - c ratio. Three of the adult subjects and one of the children had a V max-p/V max-c less than 1.0. Following inhalation of nebulized bronchodilators, none of the children or adults had a Vmax - p/Vmax - c ratio less than 1.0. Reasons for Vmax - p to be higher than Vmax - c include bronchoconstriction, decreased lung elastic recoil following a deep inspiration, and time dependence of maximum expiratory flow due to lung inhomogeneity. We conclude that asthmatic patients with spontaneous bronchospasm are only seldom able to obtain bronchodilation following a deep inspiration.