A 50-year-old woman with abnormal shadows in her chest radiograph was admitted for a more detailed examination in April 2000. A chest CT scan showed infiltrates surrounded by ground-glass opacities in the right upper lobe, and ground-glass opacities with partial infiltrative changes in the left lower lobe. A diagnosis of adenocarcinoma like bronchioloalveolar carcinoma (BAC) was made for the lesion in the right upper lobe by means of a transbronchial biopsy. However, the specimen from the left lower lobe showed nonspecific changes, and bronchiolitis obliterans organizing pneumonia (BOOP) or a metastatic lesion from the BAC was suspected. We prescribed steroids for the short term. Since the lesion in the left lobe disappeared, we performed a right upper lobectomy. In cases of primary lung carcinoma, BOOP in the opposite lung is rare and it was difficult to decide on the operability. We concluded that steroid therapy is useful for distinguishing between inflammatory and tumorous lesions in cases in which a detailed examination is difficult.