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OBJECTIVE We aimed to discuss the underlying oncological issues in staging of mediastinal lymph node metastasis in patients with left lung cancer who underwent extended radical lymphadenectomy (ERL). METHODS This multi-institutional retrospective study analyzed 116 patients with left non-small-cell lung cancer who underwent bilateral paratracheal lymph node dissection (ERL) via median sternotomy. The clinicopathological records of patients with mediastinal lymph node metastasis were examined for prognostic factors, including age, sex, histology, tumor size, cN number, preoperative data, metastatic stations (number and distribution), pT, and adjuvant chemotherapy. RESULTS Mediastinal lymph node metastases were found in 43 patients, and right paratracheal lymph node metastases (pN3) were found in 13 patients. The 5-year overall survival rate was 25.2% in patients with pN3 tumors (n = 13) and 23.1% in patients with pN2 tumors (n = 30). The prognosis did not differ between patients with pN3 and pN2. Univariate analyses showed that histology, cN, and adjuvant chemotherapy were significant prognostic factors in patients with mediastinal lymph node metastasis. In these 43 patients, cN and adjuvant chemotherapy were significant independent prognostic factors in multivariate analysis. CONCLUSIONS The prognostic factors for left lung cancer with mediastinal lymph node metastasis were cN status and adjuvant chemotherapy, and not pN status (pN2 or pN3). We hope that the study results, which suggest that there may be no difference in prognosis between pN2 and pN3, would broaden the discussion of oncological issues in the staging of mediastinal lymph node metastasis of left lung cancer.