A 55-year-old woman was admitted to our hospital with progressive dyspnea that had begun one month before. Chest rentogenogram revealed groundglass appearance and reticular shadows bilaterally. Pulmonary function tests showed both decreased vital capacity and diffusing capacity. Bronchoalveolar lavage fluid had a high lymphocyte fraction with a low CD4+/CD8+ ratio. Thoracoscopic lung biopsy revealed thick, fibro-edematous interstitium and diffuse infiltration of lymphocytes. We also observed an intra-alveolar exudate with infiltration of histiocytes and lymphocytes. The clinical features and pathological findings were consistent with subacute interstitial pneumonia, which was the entity proposed by Kawabata and colleagues. The patient developed acute respiratory failure four days after lung biopsy and died despite steroid pulse therapy. Although subacute interstitial pneumonia has been reported to respond to steroid therapy, and to have a good prognosis, we believe that subacute interstitial pneumonia could fatally worsen when associated with lung biopsy, infection, or some other stimulus.