Pulmonary blastoma is a rare primary lung neoplasm composed of a mixture of immature mesenchyme and epithelium that morphologically mimics embryonal pulmonary structure. Microscopically, tumors could be divided into two classes; those composed solely of malignant glands of epithelial components (well-differentiated fetal adenocarcinoma) and those with biphasic appearance. A 51-year-old man was admitted with left sided pneumothorax. After closed thoracostomy, chest X-ray showed a circular mass in the left lower zone. The tumor was diagnosed as adenocarcinoma as a result of percutaneous needle aspiration biopsy, and left lower lobectomy was performed. The patient was diagnosed as having biphasic pulmonary blastoma on postoperative histological assessment. This case is the first reported patient with biphasic pulmonary blastoma detected by pneumothorax.