Whether the disproportional growth of airways relative to lung parenchyma (dysanapsis) has any relationship to the development of non-specific bronchial hyperresponsiveness and atopy was investigated in 45 family members of the patients with atopic asthma. As indices of pulmonary dysanapsis, forced expiratory flow25-75/forced vital capacity (FEF25-75/FVC) and the tracheal cross sectional area divided by the forced expiratory volume (X-SA/FVC) were examined. As an index of non-specific airway responsiveness, the cumulative dose of inhaled methacholine needed to induce 35% reduction of respiratory conductance (PD35) was determined by continuous respiratory resistance measurement. For examination of atopy, skin prick tests were conducted, and total serum IgE and IgE specific to common inhaled antigens were measured. FEF25-75/FVC showed no significant correlation to FVC but showed a significant correlation to log(PD35). When the analysis was done in the subjects whose FEV1/FVC was more than 0.8, FEF25-75/FVC showed a significant negative correlation to FVC but lost its correlation to log(PD35). X-SA/FVC showed a significant negative correlation to FVC but had no significant correlation to log(PD35). These relations were conserved when the analysis was done in subjects without airway obstruction. In addition, FEV1/FVC had a significant correlation to log(PD35) and FEF25-75/FVC. However, subjects who had a positive IgE(MAST) had a significantly smaller X-SA/FVC than those with a negative IgE(MAST) (0.60 +/- 0.14[SD] and 0.72 +/- 0.18, respectively, P < 0.02). These results suggest that although pulmonary dysanapsis does not have a significant relation to airway responsiveness to inhaled methacholine, it may be associated with sensitization to airborne antigens.