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Introduction: Single-station N2 (ssN2) versus multi-station N2 has been used as a selection criterion for treatment recommendations between surgical versus non-surgical multimodality treatment in stage III-N2 NSCLC. We hypothesized that clinical staging would be susceptible to upstaging on pathologic staging and, therefore, challenge this practice. Methods: A retrospective study of prospectively collected routine clinical data for patients with stage III-N2 NSCLC that had completed computed tomography (CT), positron emission tomography (PET), and staging endobronchial ultrasound (EBUS) and had been confirmed clinical stage III-ssN2 at multidisciplinary team discussion and went on to complete surgical resection as the first treatment to provide pathologic staging. The study was completed in two cohorts (A) across a single cancer alliance in England (Greater Manchester) January 1, 2015 to December 31, 2018 and (B) across five United Kingdom centers to validate the findings in part A January 1, 2016 to December 31, 2020. Results: A total of 115 patients met the inclusion criteria across cohort A (56 patients) and cohort B (59 patients) across 15 United Kingdom hospitals. The proportion of cases in which clinical stage III-ssN2 was upstaged to pathologic stage III-multi-station N2 was 34% (19 of 56) in cohort A, 32% in cohort B (19 of 59), and 33% across the combined study cohort (38 of 115). Most patients had a single radiologically abnormal lymph node on CT and PET (88%, 105 of 115). In the majority, the reasons for missed N2 disease on staging EBUS were due to inaccessible (stations 5, 6, 8, 9) N2 nodes at EBUS (34%, 13 of 38) and accessible lymph nodes not sampled during staging EBUS as not meeting sampling threshold (40%, 15 of 38) rather than false-negative sampling during EBUS (26%, 10 of 38). Conclusions: During multidisciplinary team discussions, clinicians must be aware that one-third of patients with stage III-ssN2 on the basis of CT, PET, and staging EBUS do not truly have ssN2 and this questions the use of this criterion to define treatment recommendations.