In order to characterize BAL (bronchoalveolar lavage) in CEP (chronic eosinophilic pneumonia) and to investigate the possible role of mast cells and tryptase in the pathogenesis of this interstitial disease, cells and tryptase levels were determined in BAL of patients with CEP and in a group of healthy controls. The results show that a statistically significant increase in tryptase concentration was found in patients with CEP compared with the healthy controls. This is the first report that shows an increase in tryptase levels in CEP and could reflect higher mast cell activation as well as larger mast cell populations in the lungs of these patients. These results strongly support the involvement of mast cells and eosinophils in the immunopathogenesis of CEP.