Bronchopleural fistulas (BPF) developed in six (7.9%) of 76 patients who underwent a pneumonectomy for treatment of lung cancer. Five patients (18.2%) underwent a right pneumonectomy and one (3.7%) a left pneumonectomy. All patients were male, had squamous cell carcinoma, and were diagnosed as having BPF within one month after pneumonectomy. Their average age was 60.2 years. Univariate analyses related to development of BPF showed that significant risk factors were preoperative infection (Chi-square test; p < 0.001), right pneumonectomy (Chi-square test; p < 0.05), and metastasis to a subcarinal lymph node (Chi-square test; p < 0.05). However, sex, age, operating time, amount of blood loss during surgery, amount of blood transfused during surgery, history of smoking, degree of lymph node dissection, degree of curability, performance of combined resection, histologic type of tumor, tumor size, presence of residual tumor at the bronchial stump, and suturing method were not significant risk factors for development of BPF. Our stepwise regression analysis related to development of BPF showed that preoperative infection, metastasis to a subcarinal lymph node, right pneumonectomy, and combined resection were significant risk factors. Sometimes it is difficult to preserve the bronchial arteries upon the dissection of metastatic subcarinal lymph nodes which tightly adhere to the bronchial sheath. Moreover, after a conventional right pneumonectomy, the bronchial stump protrudes into the pleural cavity and is not covered by any tissue. Ligation of the bronchial arteries or protrusion of the bronchial stump into the right pleural cavity reduces the blood supply to the bronchial stump to a very low level and causes development of BPF. Therefore, we suggest that control of preoperative infection, wrapping of the bronchial stump, and preservation of the bronchial arteries during mediastinal lymph node dissection are important to prevent development of BPF.