We studied the perception of breathlessness as a function of air flow obstruction in 45 asthmatic subjects using a category scaling technique. Air flow obstruction and breathlessness were induced by inhalations of histamine acid phosphate in twofold-increasing concentrations from 0.03 to 16 mg/ml. The FEV1 was measured after each inhalation of histamine, and the subject was asked to rate his symptoms of breathlessness. The results showed that breathlessness increased as the FEV1 decreased but, despite a close linear relationship in all subjects (mean r = 0.88 +/- 0.15 SD), there was a considerable variation in the severity of breathlessness for any particular degree of air flow obstruction (mean intercept, 0.50 +/- 0.89 SD). However, the increase in sensory magnitude with increasing air flow obstruction did not show the same degree of variability (mean slope, 0.13 +/- 0.06 SD). The variability in breathlessness in asthmatic patients is likely to have many components. Two of these components were identified. First, we found that there was less respiratory distress in those subjects in whom air flow obstruction was present at the onset of the study. However, the presence of air flow obstruction at this time does not result in a reduction in sensation for further increases in air flow obstruction as might be expected from discrimination studies. Second, there was a significant relationship (p less than 0.01) between bronchial responsiveness and the magnitude of respiratory distress. Those subjects highly responsive to histamine experienced less respiratory distress than the less responsive subjects.