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Purpose: Lymph node involvement is one of the important prognostic factors of patients with lung adenocarcinoma. In the tumor, node, and metastasis classification, lymph node involvement is categorized only according to the anatomical station and not the involvement pattern. The aim of this study was to investigate which morphological pattern of lymph node involvement affects the prognosis of patients with surgically resected lung adenocarcinoma. Methods: We retrospectively reviewed 168 consecutive patients who underwent surgical resection for primary lung adenocarcinoma with lymph node involvement. The morphological patterns of lymph node involvement (tumor area, number of metastatic lymph nodes, presence of necrosis, and extranodal extension) were histologically examined. The relationships between the patterns of lymph node involvement, clinicopathological features, and survival of patients were analyzed. Results: Eighty patients had N1 disease, and 88 patients had N2 disease. Univariate analysis revealed that invasive size, history of adjuvant chemotherapy, and presence of extranodal extension were significant prognostic factors in N1 patients, and vascular invasion, pleural invasion, presence of epidermal growth factor receptor mutation, history of adjuvant chemotherapy, and presence of extranodal extension were significant prognostic factors in N2 patients. In a bivariate analysis including other clinicopathological factors and patterns of lymph node involvement, the presence of extranodal extension was significantly associated with poor 3-year overall and recurrence-free survival of both N1 and N2 patients. Conclusions: In patients who underwent surgical resection for lung adenocarcinoma with lymph node involvement, the extranodal extension was the most important prognostic factor among morphological lymph node involvement patterns. [ABSTRACT FROM AUTHOR]