A 31-year-old woman was admitted to our hospital because of dyspnea on exertion. She had had the same symptom a few years earlier. A chest radiograph revealed ground-glass-like shadows on both sides, and the arterial PaO2 was 54 Torr when the patient was breathing room air. We first suspected hypersensitivity pneumonitis, but the dyspnea and hypoxemia did not resolve during treatment in the hospital, and returned home. She felt no obvious changes in symptoms while at home. We found no antibodies to Tricosporon cutaneum in her serum. Computed tomography of the chest revealed diffuse pan-lobular areas of high density. Bronchoalveolar lavage was done and the total cell count was 9.6 X 10(7). About three quarters (73%) of the recovered cells were lymphocytes. Examination of a transbronchial biopsy specimen showed alveolar thickening with infiltration of small mononuclear cells. No granulomas or Massons bodies were seen, and no antibodies specific to collagen-vascular disease were detected. We therefore excluded the diagnosis of hypersensitivity pneumonitis, idiopathic interstitial pneumonia and collagen-associated pneumonitis. There was no evidence of drug-induced pneumonitis. She was seropositive for HTLV-I. We suspected that this was a case of HTLV-1-associated pneumonitis.