Current asthma (variable air-flow obstruction) is often excluded by the presence of normal bronchial responsiveness. We report a patient with occupational asthma that was presumed to be caused by sensitization and exposure to toluene diisocyanate (TDI). Variable air-flow obstruction measured by peak flow rates (PFR), and symptoms of asthma reversed by salbutamol, occurred after natural exposure to TDI when methacholine bronchial responsiveness was well into the nonasthmatic range. The asthma occurred at the end of, or just after work, suggesting the occurrence of late asthmatic responses. While the patient continued at work, the late asthmatic responses became progressively more severe as methacholine responsiveness progressively increased into the asthmatic range. This suggests that, in individual subjects, the degree of bronchial responsiveness is a determinant of the severity of the late asthmatic response. When the patient stopped work, spontaneous symptoms of asthma and increased diurnal variation of PFR recurred spontaneously until methacholine responsiveness returned into the normal range. These observations indicate that asthma can occur at a time when methacholine bronchial responsiveness is normal, providing the stimulus is strong enough. They further demonstrate that the magnitude and ease of bronchoconstriction relates to the degree of methacholine responsiveness.