The pathogenic mechanism of human T-cell leukaemia virus type I (HTLV-I)-related pulmonary disease, which involves overexpression of intercellular adhesion molecule-1 (ICAM-1) in lung epithelial cells, was investigated. The supernatant of HTLV-I-infected Tax+ MT-2 and C5/MJ cells induced ICAM-1 expression on A549 cells, a human tumour cell line with the properties of alveolar epithelial cells. Neutralization of ICAM-1 partially inhibited HTLV-I-infected T-cell adhesion to A549 cells. Analysis of the ICAM-1 promoter showed that the nuclear factor-kappa B-binding site was important for supernatant-induced ICAM-1 expression. Induction of interleukin (IL)-1 alpha (IL-1α) expression in MT-2 and C5/MJ cells was observed compared with uninfected controls and HTLV-I-infected Tax-negative cell lines. The significance of IL-1α as a soluble messenger was supported by blocking the biological activities of MT-2 supernatant with an IL-1α-neutralizing mAb. Moreover, Tax and IL-1α expression was demonstrated in the bronchoalveolar lavage cells of patients with HTLV-I-related pulmonary disease. Immunohistochemistry confirmed ICAM-1 and IL-1α expression in lung epithelial cells and lymphocytes of patients with HTLV-I-related pulmonary diseases, and in a transgenic mouse model of Tax expression. These results suggest that IL-1α produced by HTLV-I-infected Tax+ T cells is crucial for ICAM-1 expression in lung epithelial cells and subsequent adhesion of lymphocytes in HTLV-I-related pulmonary diseases.