Background: The objective of this study was to determine the survival of patients who have completely resected, nonsmall-cell, stage IIIA, lung cancer from unsuspected (nonimaged) N2 disease who received adjuvant chemotherapy. Methods: This is a retrospective cohort study using a prospective database. All patients underwent positron emission tomography scan and computed tomography scan with contrast, R0 resection with complete thoracic lymphadenectomy, and had unsuspected, pathologic N2 NSCLC. Results: Between June 1998 and December 2007, there were 148 patients (89 men). The most common pulmonary resection was right upper lobectomy in 67 patients (48%), and the most common lymph node station for unsuspected N2 diseased was 4R. One hundred and thirty-seven patients (93%) received adjuvant chemotherapy and 13% received postoperative radiation as well. The overall 2- and 5-year survivals were 58% and 35%, respectively. The 5-year survival for the 98 patients with single lymph node disease compared with patients with multiple nodal involvement was 40% versus 25%, respectively (p = 0.028). The number of lymph nodes involved (p = 0.032) was an independent predictors of survival on multivariate analysis. Median follow-up was 54 months. Conclusions: The 5-year survival of patients with unsuspected N2 disease who undergo complete resection, followed by adjuvant therapy, is 35%. Patients with single station N2 disease fare better. The role for mediastinoscopy, endoscopic esophageal ultrasound with fine-needle aspirate, or endobronchial ultrasound in patients who are negative by positron emission tomography and computed tomography is unknown, since the benefit of neoadjuvant therapy in these patients is also unproven. A randomized study is needed. © 2008 The Society of Thoracic Surgeons.