The aim of this study was to examine the inter-relationships between the different effects of deep breaths and histamine provocation on airway function in patients with bronchial asthma. Group 1 consisted of 38 consecutive out-patients with newly diagnosed mild asthma, group 2 consisted of 20 patients with bronchial asthma of varying severity who were studied during clinical remission. We measured bronchial responsiveness (BR) to histamine inhalation as the dose of histamine which provoked a 20% fall in FEV1 (PD20). The fall in forced vital capacity (FVC) after inhaling the highest dose of histamine during each BR test was calculated and expressed as percentage of the value measured at baseline (delta FVC in percentage). We studied the effects of deep breaths on airway caliber in group 2 patients by comparing isovolumic flow rates on partial (P) and maximal (M) forced expiratory flow volumes curves expressed as the M/P ratio. The changes in residual volume (RV) after deep breaths (delta RV) were expressed as a percentage of the largest VC measured on the composite M and P curves. The patients in group 1 had significantly higher PD20 and lower delta FVC than patients in group 2. There was, however, no significant correlation between PD20 and delta FVC measurements in individual patients (r < 0.1, P > 0.05). The M/P ratio was significantly related to delta FVC (r = -0.6, P < 0.006). There was also a significant positive relation between the magnitude of increase in residual volume following deep breaths (delta RV) and the degree of fall in FVC following histamine inhalation (delta FVC) (r = 0.65, P = 0.001). This significant relationship between the degree of airway closure after a deep breath and airway closure after histamine challenge is a new finding. In patients with bronchial asthma, the effects of a deep breath on airway function may be indicative of the tendency for airway closure during BR testing.