The increased bronchial production of leukotriene C₄ (LTC₄) in asthma is assumed to derive from infiltrating eosinophils expressing LTC₄-synthase (LTC₄S). Multicolor immunohistofluorescence examination of bronchial cryosections from 30 treated, untreated or bronchial antigen provocated aspirin tolerant asthmatics and nine controls revealed that the dominating LTC₄S expressing cells were mast cells (>80%), and not eosinophils. Whereas 95% of the mast cells expressed high levels of LTC₄S, only 8-27% of the eosinophils expressed low levels. Image analysis revealed a significantly higher LTC₄S expression levels in mast cells than in eosinophils. The bronchial mRNA levels for LTC₄S did not correlate with the densities of LTC₄S^+ve eosinophils or mast cells. Treated asthmatics with more than 12% reversibility had significantly higher density of LTC₄S^+ve mast cells than those with less reversibility, and it correlated significantly with reduction in lung function (FEV₁-predicted), both before and after salbutamol inhalation. Thus, mucosal mast cells, and not eosinophils, were the dominating LTC₄S containing cells in both untreated and treated aspirin tolerant asthma. The density of LTC₄S^+ve mast cells correlated, moreover with both the reduction in lung function and the degree of reversibility in treated asthma.