In order to evaluate the role of neutrophils in the pathogenesis of occupational asthma (OA), 15 toluene diisocyanate (TDI)-asthma and six grain dust-asthma patients were recruited. Controls were the same number of subjects showing negative bronchoprovocation test (BPT) and six house dust mite-sensitive asthma. Bronchoscopic biopsy specimens were stained with monoclonal antibodies to mast cell (AA1), eosinophil (EG2), pan T cell (CD3) and neutrophil (NE). Serum neutrophil chemotactic activity (NCA) was measured before and 10-420 min after BPT. Sputum interleukin-8 (IL-8) and myeloperoxidase (MPO) were also measured. There was a significant increase of NE+ cells as well as AA1+ and EG2+ cells in grain dust- and TDI-asthma compared with house dust-sensitive asthma (P < 0.05). Neutrophil+ cells and AA1+ cells showed a significant correlation in TDI-asthma (r = 0.73, P = 0.02). Serum NCA was significantly increased at 10 min after BPT and decreased at 60 min in subjects with TDI-asthma. In grain dust-asthma, serum NCA increased at 30 min and decreased at 240 min after BPT (P < 0.05). Sputum IL-8 and MPO were significantly increased after BPT in both TDI- and grain dust-asthma (P < 0.05). These findings suggested that neutrophils in the lungs might contribute to bronchoconstriction induced by either TDI or grain dust. The possible involvement of IL-8 in activation of neutrophils was also suggested.