A 21-year-old man was admitted to our hospital because chest X-ray films disclosed infiltrative shadows indicative of Mycoplasma pneumonia. He experienced fever and dry cough for 2 weeks. Chest X-ray findings showed ground-glass shadows in the lower fields of both lungs. The patient was not responsive to antibiotic therapy (PAPM/BP 1 g/day and CLDM 1,200 mg/day); dyspnea advanced rapidly and spikes of fever persisted. On hospital day 5, chest computed tomographic (CT) films disclosed interstitial shadows in all lower lung fields with dense infiltration. A transbronchial lung biopsy (TBLB) was performed on day 7, and TBLB specimens demonstrated infiltration of mononuclear cells in alveolar septa and organizing exudate in alveolar ducts with polypoid granulation tissue. Bronchoalveolar lavage fluid findings revealed an increase in the total cell count and the percentage of lymphocytes. The CD 4/CD 8 ratio was normal. The findings of other laboratory tests ruled out drug-induced lung disease, infectious disease, and collagen disease. Idiopathic bronchiolitis obliterans with organizing pneumonia (BOOP) was diagnosed. Corticosteroid therapy (methyl prednisolone: 500 mg/day) was started. After 2 weeks of treatment (prednisolone: 30 mg/day), the dyspnea and fever disappeared. Chest CT films showed that the interstitial shadows had largely resolved, but that a large cystic lesion had formed rapidly in the right lower lung field (right S 6). To the best of our knowledge, no cases of BOOP complicated by cystic lesions in the healing stage have been reported to date. We speculated that polypoid granulation in a bronchiole had given rise to a check-valve mechanism. After 2 months of treatment (prednisolone: 15 mg/day), the cystic lesion disappeared. We concluded that the bronchiolar lesion of polypoid granulation had resolved in response to therapy, thus facilitating air-way communication and the release of air from the cyst.