A 58-year-old woman was admitted to our hospital complaining of fever, dry cough, and exertional dyspnea. Chest radiographs showed diffuse interstitial infiltrates in both lungs, and blood gas analysis showed moderate hypoxemia. The levels of serum KL-6 and beta-D-glucan were markedly elevated. Bronchoscopy was performed and Pneumocystis carinii was found in the BALF and transbronchial lung biopsy specimens. A positive reaction for the HTLV-I antibody was obtained, and HTLV-I proviral DNA was demonstrated. A conclusive diagnosis of Pneumocystis carinii pneumonia in a smoldering ATL patient was made on these grounds. Without specific treatment for Pneumocystis carinii pneumonia, the symptoms and abnormal chest radiographic findings disappeared. This case suggests that the fluctuation in cell-mediated immunity associated with HTLV-I carriers may allow recovery from Pneumocystis carinii pneumonia even without antipneumocystis therapy.