Background. Clinical stage affects the care of patients with nonsmall cell lung cancer. Methods. This is a prospective trial on patients with suspected resectable nonsmall cell lung cancer. All patients underwent integrated positron emission tomographic scanning and computed tomographic scanning, and all suspicious metastatic sites were investigated. A, T, N, and M status was assigned. If N2, N3 and M1 were negative, patients underwent thoracotomy and complete thoracic lymphadenectomy. Results. There were 383 patients. The accuracy of clinical staging using positron emission tomographic scanning and computed tomographic scanning was 68% and 66% for stage I, 84% and 82% for stage II, 74% and 69% for stage III, and 93% and 92% for stage IV, respectively. N2 disease was discovered in 115 patients (30%) and was most common in the subcarinal lymph node (30%). Unsuspected N2 disease occurred in 28 patients (14%) and was most common in the posterior mediastinal lymph nodes (subcarinal, 38%; posterior aortopulmonary, 15%). It was found in 9% of patients who were clinically staged I (58% in the posterior mediastinal lymph nodes) and in 26% of patients clinically staged II (86% in posterior mediastinal lymph nodes). Conclusions. Despite integrated positron emission tomographic scanning and computed tomographic scanning, clinical staging remains relatively inaccurate for patients with nonsmall cell lung cancer. Recent studies suggest adjuvant therapy for stage Ib and II nonsmall cell lung cancer; thus the impact on preoperative care is to find unsuspected N2 disease. Unsuspected N2 disease is most common in posterior mediastinal lymph nodes inaccessible by mediastinoscopy. Thus one should consider endoscopic ultrasound fine-needle aspiration, especially for patients clinically staged as I and II, even if the nodes are negative on positron emission tomographic scanning and computed tomographic scanning. © 2005 by The Society of Thoracic Surgeons.