A 64-year-old housewife was hospitalized because of cough and dyspnea. Chest X-rays on admission showed diffuse interstitial shadows and loss of lung volume, suggesting acute interstitial pneumonia. Pulmonary function tests revealed decreased vital capacity and severe hypoxemia. Bronchoalveolar lavage studies showed that total cell counts and a proportion of lymphocytes were increased. Lymphocytes increased in lavage fluid consisted mainly of T cells with an elevated CD4+/CD8+ ratio. Furthermore, these lymphocytes spontaneously proliferated and responded well to recombinant IL-2 when cultured in vitro for 5 days, suggesting that lymphocyte activation occurred in the lung. Such lavage findings and lymphocytes activation in association with the presence of HTLV-1-specific IgA antibody in lavage fluid have been demonstrated in patients with HTLV-1-associated myelopathy. In this patient positive for HTLV-1, however, it could not be determined whether the pulmonary lesions were primarily related to HTLV-1 infection, because no IgA antibody specific for HTLV-1 was demonstrated in lavage fluid. In conclusion, when pulmonary abnormalities are found in HTLV-1 carriers, we should be careful in determining whether such pulmonary involvements are etiologically related to HTLV-1 infection.