Patients with HTLV-I-associated myelopathy (HAM) show subclinical pulmonary involvement. Bronchoalveolar lavage (BAL) studies of HAM patients have shown that there is T-lymphocytosis in association with increased soluble IL-2 receptor levels in BAL fluid. Furthermore, BAL lymphocytes as well as peripheral blood lymphocytes obtained from HAM patients proliferated spontaneously when cultured in vitro. The lymphocytes proliferating were T-lymphocytes (both CD4+ cells and CD8+ cells) and released soluble IL-2 receptors in supernatant fluid. These results indicated that T-lymphocytes were activated in HAM patients. In conclusion, activated T-lymphocytes may play an important role in the development of the spinal cord and pulmonary lesions in HAM patients.