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The exact role of FasL, and particularly its soluble and membrane-bound forms, in the development of chronic ILDs and lung fibrosis has not been extensively explored. We aimed at analyzing membrane-bound FasL expression on alveolar macrophages (AM) and lymphocytes (AL) as well as soluble FasL (sFasL) levels in bronchoalveolar lavage (BAL) from ILDs patients, incl. pulmonary sarcoidosis (PS), hypersensitivity pneumonitis (HP), silicosis, asbestosis, idiopathic pulmonary fibrosis (IPF), nonspecific interstitial pneumonia (NSIP), and healthy subjects (n = 89, 12, 7, 8, 23, 6, 17, respectively). In IPF, significantly increased percentage of AM FasL+ and CD8+FasL+ cells as well as sFasL levels in BAL were found. Increased sFasL levels were also observed in HP. NSIP and asbestosis were characterized by higher AM FasL+ relative number; CD8+FasL+ population was expanded in asbestosis only. There was a significant decline in AL FasL+ percentage in PS and HP. Vital capacity was negatively correlated with sFasL levels, AM FasL+ and CD8+FasL+ cell relative count. CD4+FasL+ and CD8+FasL+ percentage strongly correlated with BAL neutrophilia, an unfavorable prognostic factor in lung fibrosis. The concurrent comparative BAL analysis of FasL expression indicates that FasL+ AM and AL (mainly Tc cells) comprise an important element of the fibrotic process, mostly in IPF. FasL might play a crucial role in other fibrosis-complicated ILDs, like NSIP and asbestosis. (Folia Histochemica et Cytobiologica 2011; Vol. 49, No. 4, pp. 636–645)