Infiltration of the lower respiratory tract by helper/inducer T lymphocytes in HTLV-1-associated adult T-cell leukemia/lymphoma. The human T-cell lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia/lymphoma (ATLL), a disorder in which peripheral blood and multiple organs are infiltrated by malignantly transformed T lymphocytes. We investigated the nature of pulmonary disease in a patient with serologic evidence of HTLV-1 infection. In this case, endobronchial biopsy specimens showed infiltration of the bronchial mucosa by pleomorphic cells exhibiting a high degree of nuclear irregularity. These cells were morphologically identical in appearance to malignant cells found in peripheral blood and infiltrating the dermis, expressed the OKT4/Leu3 phenotype and the receptor for interleukin 2, and, by analogy to gene rearrangement studies on leukemic blood cells, were monoclonal in origin. However, in situ hybridization of endobronchial biopsy specimens with full-length HTLV-1 probes failed to detect retroviral RNA or proviral DNA. These studies indicate that T lymphocytic involvement of the lower respiratory tract in HTLV-1-associated ATLL is characterized by expression of a malignant phenotype despite the inability to document actual cellular infection with this retrovirus by a molecular hybridization technique. 