Human T-lymphotropic virus type 1 (HTLV-1) carriers are known to develop pulmonary complications characterized by T-lymphocytic alveolitis. The aim of this study was to determine the profile and role of soluble Fas (sFas) and sFas ligand (sFasL) in the lung of asymptomatic HTLV-1 carriers. We measured sFas and sFasL levels in serum and bronchoalveolar lavage fluid (BALF) of 16 seropositive asymptomatic HTLV-1 carriers and 32 healthy subjects. The serum levels of both sFas and sFasL were significantly higher in HTLV-1 carriers than in the control. In BALF, the percentage of lymphocytes and CD4 positive T-cells, and the levels of sFasL were also significantly higher in asymptomatic carriers than the control, but there were no significant differences in sFas levels between the two groups. There was a significant correlation between BALF sFasL levels and serum sFasL levels and percentage of CD4 positive T-cells in BALF. Our results suggest that the increased levels of sFasL in the lung of asymptomatic HTLV-1 carriers are associated with accumulation of CD4 positive T-cells, and that resistance to apoptosis in HTLV-1 infected T-cells and overproduction of sFasL could contribute to T-lymphocytic alveolitis by down-regulating Fas-FasL mediated apoptosis.